It's mostly up to you. You just need to obviously not be fat, you know? It's about good faith representation and showing yourself in such a way that makes accusations of hambeastery impossible. It makes my job easier if you show skin, but not everyone is comfortable with that, and fair enough. Some people are in underwear and some people have a relatively tight fitting top. I think I wore a regular shirt and pulled it tight. While it should be pretty clear someone is thin even in loose clothing, wearing something baggy might suggest you are a smallfat trying to hide a proto-fupa.