Some good news some bad news.
I played around a bit in kaitai and got it somewhat working for i2d bitmaps, but only for really small ones. Larger ones seem to be chunked, LZ compressed and run length encoded, and I wasn't able to figure out uncompressing them.
The spec files mentioned above are gone for good I think, at least I wasn't able to find them anywhere (believe me , I tried). Not all is lost however, as the entire (?) Revenant source code is on github, so for someone who actually knows c++ it should be possibe to figure out the format.
Link to kaitai thingy:
https://pastebin.com/7rTFwcfd
Revenant source code:
https://github.com/benjcooley/Revenant