All the textures are labeled, and they have to be named that way to use the uploader, meaning that for the left hand, the texture would have to be named l_hand etc. For joint textures it's j_l_wrist etc.

Depending on the size of the texture you're uploading, you usually can't do more than 7 or 8 textures at a time. If you try to and it doesn't work, that's why.