You edit the GameData/Squad/Spaces/Partyouwanttoedit/fileinwhichtheinteriorisshown.(usually model001)
So in GIMP you have to make sure you export the file as a .dds (its usually a .dds) and set GENERATE MIPMAPS and compression to the first one (BC1).
It's... low quality. It can't be better. (I made the tape too)
P.S. Of course, to get to this I tried all compression methods.
Aerodynamic Kerbal's post in I wanted to put a propaganda image (I made it btw) in the Cupola module, it didnt work. was marked as the answer