The ingame objects aren't stored in one piece. there is the 3d object with all the triangles and stuff and the textures which are 2d images which are .dds but joymax added a little header and changed the name but they are just .dds
So what they do ingame is just take that picture and put it around the 3d object. I've seen some .dds files of faces and they were all streched.
So you can easily convert the .ddj to a .dds there are some tools which do this for you and you just load the .dds in photoshop and edit and convert it back to .ddj and import it back to the .pk2 file