700MB is still ok. Let the people decide if they want to use that much space.
For the non working iffs: Have you tried to load them into Deluxe Paint and just resave the image? Mostly this works fine. Thanks for the preview pack. I will test it later.
edit: I guess you didn't add black bars to the screenshots yet? Most of them are cropped. The covers doesn't look good enough without proper dithering, indeed. Some shows artifacts (e.g. Turrican 3). We need the dithering quality like the above cover from Operation Thunderbolt. Check the attachment for images Gibs already converted. This is what we need.