You know those vector arrows? Maybe use them coming from the top left going down to the bottom right. Some splatter brushes on low opacity?
Don't do either of those. a.) Vector work other than people on shirts is a big no no. b.) Splatters of paint on a clown? uh, no. It's ----ing ace as it is though. Illustrator?
looks good, i dont think it needs anything in the background...i think if you add too much it will take away from the main focus of the shirt which is the clown. I think the clown is good enough to stand alone on the shirt...good work