Brad rarely accepts art submissions. They've got to be pretty original, creative, and good for him to put them on here. Look at the DexX stuff in particular. Also, Kaddar.
Original submissions are all that's going to get through, unless you get permission from an original author, and even then it's pretty iffy. I personally think that we should lean toward purely original characters, as they create far less of a licensing headache when it comes to Stripcreator products and such.
When it all comes down to it, it's best to give Brad art that is going to be the site's, belonging mainly to Brad, but partially to everyone who uses the site. Again, some of my favorite characters to use are out of the DexX and Kaddar categories, as well as a few from the "User submitted" one.
I think we need to keep away from adding too many pieces of art that resemble other characters, since that's probably clogging up the bandwith and such, and it's just better not to be repetitive.
I bet that it would be really nice for Brad to have someone really good at coding who could occasionally add stuff like art, that way it's taken off his head, but that's not very likely, if I know Brad well enough.