I just re-discovered that the IFF variants for 16/32 or 256 colors are entirely different file formats : ILBM for the first, PBM for the second. So in Grafx2 I'll have to write the saving code from scratch... jvdw007, don't expect it too soon.
To freehand: For moving back images to the Amiga (OCS), it may help to encode them back to ILBM format while on the PC. XnView includes a command-line utility "nconvert" that can do it. It's rather helpful that it can do batch conversion 'in-place':
nconvert.exe -out iff -colors 16 yourdirectory/*.iff
But one little problem, it doesn't perform run-length compression at all, so files can be big...