(I thought I'd record the explanation of the Homestuck shirts and prints here, for a little bit of posterity and persistence.)
Ok, so this is all a little different from the usual stuff for sale. Instead of just slapping humorous or story-relevant stuff on a shirt (which is all well and fine, don't get me wrong!) I sort of wanted to take the opportunity to let this stuff tell a little story, and add some depth and detail to the Homestuck universe.
There are two character shirts available, John's and Rose's. But the shirts have their own little backstories, which are conveyed in corresponding prints, also for sale.
I think the stories sort of speak for themselves through those graphics, but I'll go ahead and overexplain anyway. John's shirt, which of course appears to be Slimer from Ghostbusters, is actually a depiction of an Asian knock-off of Slimer called the GREEN SLIME GHOST. Rose's shirt was once a depiction of a character from a fake cute cartoon show which I totally made up called "Squiddles!". She then went about defacing this shirt to make it look mean! The shirt itself actually appears to be defaced, with ink missing around the eyes and mouth area (but it's a pretty subtle effect, because the ink color is very close to the shirt color).
The cool thing about Rose's print is you can get either the faux-Lovecraftian defaced version, or the undefaced version (so you can deface it yourself!) Just make sure you select the option you want underneath: "LOVECRAFT MY SQUIDDLE!" if you want the defaced version, or "DON'T LOVECRAFT MY SQUIDDLE!" if you want it unmolested!
So there you have it. The story of the shirts, and some sweet merch all rolled into one.