Well it's called fan art. Fans will do what they want with the pictures, and given there's a lot of lonely perverts playing video games, we get slutty and downright pornographic pictures.

The problem is many anti-Tifa sorts will use these fan images as 'evidence' of Tifa being a slut, which is just bull.

I happen to be a very avid fan art/doujinshi (Japanese fan comics) collector, and I've got plenty of images of Tifa that are very tasteful (and some that arent too. I just love the art style sometimes! xD)