I always sign small in one of the corners or someplace where it fits in the picture and it isn't even noticed unless pointed out. Sometimes if i use writing in the art like in my signature - i'll forget to sign it because it already has my writing and it's proof enough. Unless it's art I post online - then I'll watermark it with my screen name on the bottom enough so if anyone ever tried cutting it out people would be able to tell that a signature WAS there before. Either way - I think it's a good idea to sign your pictures - it lets people know that you drew it and and it helps prevent art theft which is so common these days