there should be a
"painting_xxx.dds"
"painting_xxx_spec.dds"
"painting_xxx_nrm.dds"
the one you want to edit is the top one, the one with no suffixes. When you open the .dds file in photoshop, there should be a seemingly random scatter of images, ranging from gold trimmings, gold planes, and importantly - the picture. Don't worry about the other things, just focus on the actual painting. Find a painting you want to use, paste it into the .dds file as a new layer, crop it / scale it / move it until the picture matches perfectly with the old picture. Then merge visible layers, autocrop layers, and export as a modified .dds (save as new don't overwrite!)
I've never done this myself, so just ask if something doesn't work, hopefully it does!