welcome to the forum and thanks for asking first
You are allowed to use the pictures as avatars and profile pictures, this is no problem.
If you own a forum yourself and want to offer SP-Studio characters in the list of available avatars, it depends on the amount of pictures and visitors. I allow this for small forums and if it's just a few pictures. But please mention where those images come from. You don't need to write sp-studio.de on the avatars, because this looks ugly
. But maybe you can mention it in a Forum post about the new avatars or on the page where you select them. It is up to you, I just ask for some kind of credit by showing the sp-studio.de link somewhere.