Nope... not allowed.. whatever photo/anime image and so on, you use in your signature, you HAVE to get permission to use it from the artist.
Belive me, they want you to email the artists and ask if you can use it.

Its really STUPID!

You can read all about in their rules... belive me...i used anime stock i found on the net/google/screenshot - you name it....someone reported me, and voila - en empty gallery.

I sent messages to their board, and they would not budge, Thats how the rules were.

People report you by pressing the Copy violation button thing under you pic... and alert the Deviant Art crew...