RW-Tools works like a charm.
1. Open myfile.tgpcdx; it should be readable (even if it's a bin file)
2. Go to the Graphics tab : choose conversion .tgpcdx to .dds. This will produce a small window giving 3 key infos : dtx file format, and mip map level. Do write it to a safe place(in case of problem)
... this step will produce another important file aside the .dds file : myfile.dat (an editable ascii file). Never erase it as it is key to convert .dds back to .tgpcdx.
*** when saving your repaint, let's say from Photoshop, save the new .dds with the 3 parameters given in step 2 and *** name it *** the same the as the .dat file name : thus myfile.dds
This last conversion will AUTOMATICALLY place the new .tgpcdx file in the proper Texture folder ....
Good luck, QcRail