Several shirt icons appear in Homestuck, all of them are shown on one of the 8 Homestuck Kids' shirts, as well as on the trolls' shirts. As an example, John Egbert has Slimer, from Ghostbusters, on his first shirt.
The shirt icons are seen a bunch of places, and change every so often. Jade even has a machine to change it for her.
Main icons Edit
John, Jade, Dave, and Rose Edit
Jade has a blue Slimey, a blue Homestuck Sun, a blue pumpkin, a blue Leaf, a blue SBURB Icon, a blue Squiddle, a blue Flower, a blue atom, a Prospit Moon, a Star, a Heart, a Horseshoe, the space aspect symbol, and God Dog on her shirts.
Rose has a (slightly altered) Squiddle, the light aspect symbol, and a Derse Moon icon on her shirts.
==== Jake, Jane, Dirk, and Roxy ====
Jake has an emerald skull, a Puppet, a Prospit Moon, and the hope aspect symbol on his shirts.
Jane has a different kind of Slimey, a Prospit Moon, and the life aspect symbol on her shirts.
Dirk has a Cap, a Propeller Hat, a Derse Moon, and the heart aspect symbol on his shirts.
Roxy has a Mutant Cat, a Derse Moon, and the void aspect symbol on her shirts.
You may have noticed that each of the humans has a moon listed as an icon. This refers to their dream self's shirt on either Prospit or Derse.