I just had a thought about the question being raised,"... to use my image in his app?"
I hope that I am not opening a can-of-worms but... In most cases forums of this type generally assume that all media submitted, unless otherwise stated by the user, would be that all media is at least public domain. As such are free to be used by one and all. If the user submitting the media explicitly does not give permission to use said media, then permission 'must' be sought and the appropriate credit is added to the intended program. If tyhe answer is still, "No." then end of story. Look for public domain media or make it yourself.
Regardless of whether the media is public or owned the best thing to do would be to simply ask if the media can be used.
Please don't shoot the messenger....