For my Illustration type stuff with pen and ink and then finished in photoshop I generally sign the work... You'll see the sig in the right corner. very discreet. I don't think I've ever signed a poser image yet but I'm relatively new to the medium sorta. Copyright of original work in the internet age is a complicated thing.

click image for larger view.

(image is very large in it's original .psd file, so there's alot of compression in .jpg format)