IMVHO there is no need for categories. I just browse the gallery without selecting any category, so I don't miss anything. they may have started enforcing the "mixed medium" category after complaints from some folks, who alleged that one artist was doing photomanips, then categorising them as poser images. I never saw any evidence to support those allegations (e.g. an actual photo that the artist may have photoshopped to resemble a poser render), but he was encouraged to relabel them as "mixed medium" nonetheless.