J.K. Rowling regularly speaks against this sort of thing. It's one thing to photshop women as a matter of course to increase their bust size and thin their waist. Not that it's not immoral with adult women, but it seems to me to be a completely different matter to do it with someone who is underage (just turned 17, probably 16 when she took the picture) who is portraying someone even more underage (15 at the beginning of the movie, 16 at the end).
Several of the commenters have already made this point, but I'll make it again here. If whoever was responsible for this perverse act doesn't think Emma Watson is attractive enough to teenagagers as she is, then our culture's standards of beauty have become even more warped than I had thought (and I've long thought them to be pretty twisted). We already tell girls in too many ways that they're not good enough unless they look like Emma Watson. Now it turns out even Emma Watson isn't even good enough as she is.
Update: More here. I've also now linked above to Rowling's own rant against this sort of thing.
Update 2: Warner Brothers claims that they didn't authorize this. They've asked IMAX to remove it from their site.