Since filing this request, deark has added support for all the 'known' RGFX files that we could locate. Since there is a modern converter now for this format, it's far less priority to add it into recoil. So you could close this request if you'd like :)
By the way, I had asked about this support for deark a while ago: https://github.com/jsummers/deark/issues/47 Sadly it wasn't able to be added due to not the compression being XPK based and that being a big thing to tackle. Perhaps it's out of scope for recoil too. This code supports many XPK compressions schemes, so you could leverage that for the XPK decompressiong if libancient.so is available on ths system recoil is being built on: https://github.com/temisu/ancient
By the way, I had asked about this support for deark a while ago: https://github.com/jsummers/deark/issues/47 Sadly it wasn't able to be added due to not the compression being XPK based and that being a big thing to tackle. Perhaps it's out of scope for recoil too.
IFF-RGFX support
Problematic RAG file
Problematic Apple IIGS files
Found 3 more files that don't convert well from the same source CD. 1 SPS and 2 SPC.
These files come from this CD: https://archive.org/details/atari-gemini They are in a 'specpics' folder.