Hmm, interesting, it appears that some of the GIMs in there are using DXT5 colour format, which is somewhat non-standard, which might explain why they won't work when converted to PNG then back to GIM.

The following images in xmb_plugin_normal.rco use DXT5 colour:

• tex_indi_chat
  
• tex_indi_message
  
• tex_indi_online
  
• tex_indi_online

If you don't need to edit them, you can dump without converting and manually convert the remaining three images (or do 2 dumps and fix up the XML).

If you want to edit those images though, you can try manually converting them to GIM, forcing Gimconv to use DXT5 colour, using a command line like:

for each image.  Then reference these GIM files in the XML so Rcomage can merge it.

Hope that helps.