While I don't have experience uploading to ShareCG, I can say that I've seen a number of textures for 3D models offered as a standalone download. I think putting into a Zip or similar is strongly encouraged. However, I've seen some as straight .jpg. Anyway, .jpg is already quite compressed and it's unlikely .zip could compress it much further (if at all). I would suggest, though, that if you plan to create more textures for the same model, it would be considered polite to put them together as a single download, rather than on separate pages/downloads.

As for the category: If it's a texture for a 3D model, that falls under 3D Model. Though, if it's a texture for a DAZ model, would that be considered the DAZ Model category, instead?