I dont think much about it unless I'm being shopped on nudes then again if that happens I probably would try not to pay much attention.I can see how it'd be flattering or funny; but having your photos manipulated by some anonymous internet person would have to be violating to some extent, even - and possibly especially - in the case of ridiculous fetish work.
I'm still sort of morbidly curious as to how someone would go about retouching something of me. Though in that case, I probably just want the validation of being considered for it in the first place.