Is it wrong

Would it be considered inappropriate to comment on the photo that they have posted on how great someone looks in there new outfit?

What like profile picture/display pictures?
A lot of us comment on each other's pics, in an absolutely non-pervy, appreciative way :)

We all love compliments; I would find it flattering.

then can we message the individual?

I think it depends. The rules kinda state no one-on-one flirting, or using graphic displays of affection and unnecessarily graphic comments.

So I guess saying "That new outfit looks amazing on you" is okay....but it is crossing the line if your comment has a more seedy "I am perving at you right now" undertone, or hints that their image is arousing you etc.

Sadly not. The best way is to comment when following on from one of their posts.

Obviously it should be a general comment, not an overtly sexual one.