This is what a Jacko looks like.
Jackos are pictures of ComJay that appear in hidden locations inside his infamous game, The Brilliant Game. Said picture is first seen as the game's file icon, which shows up when you load up the game to play.
Right after starting the game, the creator challenges the player in finding 5 hidden Jackos, which are scattered around the map. If the player manages to find all of them, it will get a bonus. The player will only get it by taking a screenshot of all of the Jackos and contacting ComJay by e-mail and showing him these screenshots to prove you found them all;
It's good to note that Jackos aren't even items in the first place, so nothing can be actually done with them in-game once they're found. Basically, all they do is play a little music once you get too close.