I was talking about basic string hashing (not images), so I guess that would be even simpler.

> I can retroactively adjust the hex digits in the image without affecting the resultant hash

My point is: I give you a hash H, can you generate a collision by finding a string X, so that when I append an (unknown to you) salt S then MD5(X + S) = H.

EDIT: To make it clear, the only feedback you get when you try X is whether the final hash matches, you don't get the resulting hash each time.