Okay , its only identity theft if : you are using the girls credentials as well. That girls actual name birth date etc. or using info's to transfer out of her account (s) It is ok on fb to use avatar that is not you , unless you uploaded to self portraits a pic that doesn't have your face, but , that is : a hyper specific non enforced fb rule.
The same person making multiple accounts through separate emails is a big fb no no. A strict rule with them , for sure. However , at this point , only violating fb rules heavily , not yet technically law breaking.
You have the benefit of the doubt of being : just someone who saw a pretty pic they liked and used it as an avatar , and not understanding fb rules about not making alts.
The illegal comes into place when/if : you are using that account to make adult content referrals.
Long story , my first thoughts of online money involved : adult content. My research was in the understanding of adult content rules. You need docs and permission for any person appearing in adult content OR promotion thereof. This law is similar in many countries.
Photocopy of a picture id for the model appearing, and proof of permission.
The only way this rule is bent is if : the site your affiliated with gave you a stock of images , then the liability is on them.
I know Facebook is a big all encompassing soul eating site , but , they don't have any more legal authority than any other web based services. Its the same legally as trolling a minor message board/forum site.