You should specify what period you are referring to, whether you meean the 6th-5th centures, or the 4th century and afterwards. The early period is easy to guess but the later period is where there is likely to be argument. There are some pictures in Duncan Head's Armies of the Macedonian and Punic Wars IIRC and one of them plus more coin pictures are here:
The Paeonians and Agrianians. You can buy 28mm figurines of those guys, too.
There are also some pictures in the Osprey books on Alexander and you can get some ideas also from my Osprey book on the Thracians (The Thracians 700 BC-46 AD, Osprey 2001)
Here's what I wrote in
"The Gods of Battle" (Pen & Sword Books 2012)
The Agrianians
Herodotus says that the Agrianians lived north of Mt Pangaeum and when the other Paeonian tribes were forcibly removed to Asia, the Agrianians stayed behind as the Persians couldn’t get to them. They sent troops to join Sitalkes’ invasion of Macedonia. Arrian (Alexander, 7) said they were one of the four most warlike and robust peoples in Europe (the other three being the Thracians, Paeonians, and Illyrians). The Agrianes were a Paionian tribe not included in the Paionian state, but with a king of their own. They provided light infantry to Macedon, but (unlike other Paeonian tribes) no cavalry are mentioned except by Curtius (4.15.21) who is most probably mistaken (probably confusing them because they were acting in their hamippoi role).
Their fame comes from their use by Alexander the Great as his Special Forces troops, his own SAS (or Special Agriane Service) regiment. They were probably supplied to Alexander by the client king of the Agrianians, Langarus, out of his household troops. King Langarus was offered Alexander’s half-sister in marriage, and said he would fight the Illyrians for him, but died before either could occur. Only one company (probably 500 strong) was present at the crossing of the Hellespont, but a second company joined the army before Issus, bringing their strength up to 1,000 (under the command of the Macedonian Attalus). The Agrianians were still 1,000 strong during the invasion of India (Arrian 4.24). They were armed with javelins and a sword or knife, but probably also knew how to use a long spear or a bow if necessary. At the beginning of the battle of the Hydaspes, the Agrianes are called “spearmen” while others are described as “javelin men”. Later in the battle, they are using javelins as Curtius (8.14.24) says they “unleashed a thick barrage of missiles on both elephants and drivers.”
They probably also carried a shield and wore a helmet, depending on the requirements of the mission. Given their elite status and their importance to Alexander’s battle plans it is very that Alexander equipped them with better armour and weapons when it became possible. They were not mentioned in frontal assaults, which are usually given to Alexander’s Guards, so it seems likely they wore only light armour. Moreover, at the battle of the river Hydaspes, Curtius (8.14.24) says they were “better at skirmishing than fighting at close quarters” Arrian III: 23 says something similar:
..the Agrianes, who formed the rearguard, had been attacked by the hill tribes. However, their long-range fighting was too good for the enemy, who had the worst of it and withdrew.
The Agrianians were always in the thick of Alexander's battles, and were often called upon to perform special roles. They were the crack light infantry unit of the army and some of the best light infantry in the ancient world. If there was a particularly difficult mission or one that required fast moving troops, Alexander got the Agrianians to do it. For instance, they were part of a force that marched 75 miles in two days and surprised the Arians (Arrian 3.25). This rate of advance suggests that they may have been mounted for longer journeys, perhaps riding double with the cavalry with which they so frequently operated. During the siege of Tyre Alexander firstly took “some squadrons of cavalry, the Agrianians and archers” (Arrian 2.10) for a quick raid into Arabia; then took with him only “the shield bearing guards and the Agrianians” to go to Sidon (to get some triremes)” (Arrian 2.19). At the Persian Gates the Agrianians formed one of the largest contingents for the crucial flank attack. They force marched 12 miles through the night along a rough and narrow path to surprise (from the rear) the Persians defending the pass. It may be safely assumed that the Agrianians were amongst the 300 men who “in previous sieges had experience of rock climbing” and climbed the Sogdian Rock using tent pegs and ropes. At the Rock of Aornos they were part of the small select force that advanced up a rough and difficult track and seized an advance post that enabled the eventual capture of the fortress (Arrian 4.29). Curtius (8.11.9) says they also scaled steep cliffs as part of that assault. At the siege of Cyropolis, in Bactria, the defenders were distracted while Alexander, with the Agrianians, the Guards (hypaspists), the archers, and his personal guard (only seven men more), crawled under the walls in a few places where a stream (which was dry) flowed under the walls. They then opened the gates and let in the attackers (Arrian 4.3). That was one of the numerous times when they were under Alexander’s personal command. For the attack on the Uxians (Curtius 5.3.6) they traversed a secret craggy and mountainous path at night and emerged behind the citadel, enabling it to be conquered.
They were also on the right flank in nearly all the battles, usually brigaded near the Companions or Royal Guards and acting as hamippoi. For instance at the battle of the Granicus, they were placed amongst the lead troops of the right wing. There they caused mayhem by making their way in amongst the Persian horsemen and hamstringing the horses. At the battle for the Shipka Pass, the Agrianians were at the forefront of the charge up the slope that routed the Thracians. At the battle of Issus, they formed the extreme right flank of Alexander’s army, and charged and routed several times their own number of Persian light troops on higher ground in the mountains. As soon as the Persians retreated, Alexander recalled the Agrianians to strengthen his main attacking force i.e. his companion cavalry and usual right wing troops.
At Gaugamela, they were split into two units, one brigaded on the extreme right with the Royal Squadron, The other half are stationed immediately in front of that squadron and the rest of the Companion Cavalry. The right-wing unit showed their bravery by seeing off the scythed chariots. They “hurled their javelins at some of the horses; others they seized by the reins and pulled the drivers off, and standing round the horses killed them.” (Arrian 12-13). Curtius (4.15.21) says that at one stage in the battle, Alexander was surrounded by Persian cavalry and in terrible danger, but the Agrianians cut into their rear, assaulted the Persians, and forced them to turn around and defend themselves from the Agrianians instead.
At the Tanais river crossing, the Agrianians were with the lead battalions, firstly helping to fight off the Skythian horse archers, then mixed up with the cavalry that attacked and routed the Skythians (Arrian 4.4). When advancing rapidly through the “rough and mountainous” territory of modern Bajur and Swat (in Pakistan), to assault the towns there, Alexander took the Agrianians, as part of his small select strike force (Guards, Companions, foot companions, archers, and mounted javelinmen) (Arrian 4.23). At the town of Massaga, the mounted javelinmen, the Agrianians, and the archers were the first to go into action, after feigning retreat. Curtius (9.8.18) says they also feigned retreat before the walls of another Indian town (between the Musicani and Patalia). Both times the Indians were provoked into a disorderly pursuit outside their town, and defeated.
At the Hydaspes (Jhelum) the Agrianes are taken across the river with the king and his hypaspists. They take their position on the right wing as usual. (Arrian 5.13- 14) Curtius says they were instrumental in defeating the Indian elephants.
They were again brigaded with the guards and elite cavalry on the right wing at Sangala (5.23-4). Afterwards, they were assigned, along with the three guards regiments and the archers, to a special force that, after working all night, stopped the Indians from leaving the town. The Agrianians were often under Alexander’s personal command, as when they were put with the Guards, the archers, and the “picked cavalry squadron” for the voyage down the Hydaspes and Indus rivers (Arrian 6.2). They were thus the first to arrive after the cavalry for the attack on the Mallians (6.8). The Agrianians have not ever received the recognition due them for their part in the conquest of the Macedonian empire.