These two shirts are 'borderline' shirts, once again from SharkRobot. They aren't particularly licensed/copyrighted it seems, but are also somewhat generic in some way to skirt past that. At left is a "Vortex Wisp" from Sonic Colors (that one that 'eats' stage obstacles & etc when activated. It's basically just a 'scary face' though, so it becomes sort of an inside joke on the tee, with the words "OM NOM NOM" written next to it, and some paint spatter effect. The right shirt is "A Wordy Hero" which uses various "Sonic Describing" words colored & shaped together to make a sort of image of him. Notice the expression isn't spot on, and neither are the hands, as official art doesn't seem to have been used for this. The bottom right shirt shows a glow/sihlouette type 'back of classic Sonic' with a large ring, so it's somewhat abstract. These are all creative designs which you can find at www.sharkrobot.com. 

Discovered by Taaron