The other evidence that it's definitely not Brooke Shields is an interview she gave with Nylon in 2000.
I don't know if it's reverse psychology, but my mom would say, "You wanna do drugs? Fine. Do them. Just do me a favor. Let me do 'em with you. I'll get you the best stuff. This way I know you're not going to die." And the whole thing was so unappealing to me.
Note: no one has ever lied to Nylon.
Honestly, at this point it could go either way. What's most interesting is how vehemently Shields' representation has denied everything, as though the existence of an old photo would have any negative effect on the actress' career.
At the same time, no one likes being accused of something they didn't do. So no, that's not Brooke Shields in the photo, unless her rep is lying, in which case it probably is.