A few people have pointed out that it's the fact that the pictures were intended to be seen by their unwitting subjects - it wouldn't be as bad if they'd been drawn for the amusement of others only, and would be even less bad if the subjects had asked to be drawn. As was said so well upthread, there's a distasteful "let's take this bitch down a peg or two" undertone both to the pictures and the egging-on that accompanies them.
Ssly if you're someone who needs it explained to you why those pictures are wrong, I doubt you'll ever agree. I don't think empathy can be learned, once you get past your teenage years. We're probably all on a sliding scale, with total sociopaths at one end and people who can't cope with mild peril in Mary Poppins at the other. Some people will look at these pictures and cringe in sympathy for their target. Other people will think it's funny because it's not happening to them. Others still will laugh in a "know it's wrong but can't help myself" sort of way.
I am sure though that neither the original artist nor the copycats he spawned would approach a woman in the street, or a woman they knew, with a picture like that. "Hi! Here, I drew you this picture. Look, I put your head on Jabba The Hut's body. In a dress. Because, you're, you're, a bit fat. Do you see. It's just...yeah. Anyway, I worked pretty hard on the detailing and everything". Or "Hello! I think you look a bit like Chris Eubank, you know - the boxer? So - here. I did this for you. I hope you like it". It's easier to do from behind a screen, when you don't have to deal with your victim's reaction - partly because you can hide, partly because a little part of your monkey-brain doesn't believe there's a person at the other end anyway.
I have spent far too long thinking about this - but it has been bothering me today. This and that story on the news, the one about the guy who (alledgedly) shot the Indian student in the street, whose dad had to find out about it on Facebook? Who strutted into the courtroom today and called himself "Psycho" Anderson or whatever. The pistonheads thread and that story have sort of curdled together into a "people are shits" angry cloud in my head, which I accept is not the most rational way to approach things.