I do think that it's both rude and dishonest to post pictures that look nothing like you anymore (as PPs have said, a noticeable weight gain or loss, or a picture that's years out of date). Like other people mentioned, if I met someone whose photo showed him to be of average weight and in their mid-twenties, and he turned out to be 100 lbs overweight and in his mid-thirties, I'd wonder what else he was lying about. I suppose it's possible
that he just didn't realize how much he'd changed, but with something that major it seems unlikely. I'd assume they were trading on the assumption that "Well, I may be very overweight and 10 years older than my picture, but if I can just get to the first date, she'll see how awesome my personality is and not mind!" Actually, while the weight/age might not turn me off if he was honest about it and seemed like a great guy in other respects, the fact that he actively disguised it would bother me far more than the extra pounds.
However, having a picture that looks a bit different from the way you really do (something about camera angles, maybe), or you're wearing glasses when you normally wear contacts, or a guy has a beard that he recently shaved off, I think that's all right. It might be good to give a head's up before you meet ("Oh, and I've grown a goatee/cut my hair short since that picture was taken), but it's not as big a deal as a really out-of-date picture. And I do think full-body shots are good, as well as having more that one picture, just to get a good idea of what they really look like. I had a basic head-and-shoulders picture, as well as a few others (me dressed up like Malcolm Reynolds from Firefly
for Halloween, a cool action shot of me on horseback at full gallop, I think maybe another one I don't remember).
I met my BF online, and his picture, while a nice one, just doesn't look all that much like him. I think it's the angle it's taken at; there's nothing I can point to exactly that's off, but it doesn't look quite right. Honestly, when we first met, I recognized him from his smile, which is wonderful and does show up in the picture