RECOIL already support multicolor Image System Files, but not the hires variant. The load address is 0x4000, followed by the bitmap data, 192 bytes of unknown data (just as with the multicolor variant) and the screen data. Summing up to 9194 bytes. (Information found at Codebase 64.)
Please attach a sample file.
I don't have any file created with the actual Image System application. So I don't know if the 192 unknown bytes should actually contain something other than zeros (but it shouldn't affect the decoding in any case). I attach one file converted with tomseditor.com (which doesn't even try to preserve any color and seems to get the load address wrong) and one converted with pyriel. Both loads fine in tomseditor.
Implemented, thanks!