from wich i know, if you paid for an asset that's also the license it provides. So if you have receipt and the other developer want to bother you he will have to talk to the designer. The designer could be sued but not you.
For apple any trouble about developer will for sure block you from the market, they only care about money.
I do have licence & receipt but my concern was about one another developer who used same graphic for similar game. So that is what is really bothering me.