Thanks so much yrizoud! grafx2 is such a fantastic tool, I'd be glad to be of any help I can to make it better.
What I don't uinderstand is this: in grafx2 you can set the image size (canvas) and screen resolution of the software. I don't think you should save at all the screen resolution of the software, rather, just save the information of the canvas used. If I need a 320x512 screen, i should take the work to specify that on grafx2 and then DPIV and others will read it accordingly.
Since grafx2 is not necessarily switching to these wack graphic modes (and there's no need to in today's computers), saving the screen resolution grafx2 is using to the IFF file seems unnecessary and, as we can see, counter-productive.
Do you also think it is possible to really limit the colour numbers that grafx2 saves on an IFF file? I don';t even know if such files would be readable by DP3 or other software. As it is, for example if yo are developing a game, you will always need DPIV or similar to make the final save of your files for proper Amiga use, because grafx2 files are not directly usable.