Also PaintShopPro[not Photoshop] lets you save in iff format if you can get a copy but gifs will generally do.
Photoshop can save in .iff, I used to use it all the time when I wanted to quickly make custom wallpapers to fit a "high res overscan laced" Workbench screensize. Hard to find native 702x482 images otherwise, lol.

It's right in the dropdown, might depend on your version, dunno.
You make a lot of work for yourself with all those steps, but to each their own. Can't say I didn't do similar, back in the day.

These days Ken's Icons are good enough for me, but obviously those only look good on 16-bit screenmodes or better.
3.9 will run with 6MB fast/2MB chip, but personally I wouldn't do it with less than 16MB.