For me I only post my LP sigs And I'll usually put them under photomanipulation and their desired placeunder that sub
it depends on how you make it....if you do a digital painting that just happens to be the size of a sig then you would put it with other digital paintings. same as if it were a photo manip sig or a 3d one
I suggest you don't upload signatures at all. Reasons: 1. Most people consider it as unfinished art and don't take the time to comment it. 2. In 90% of all the cases they get removed by deviantART due Policy Violation because the renders/stock images are copyrighted material.