Hi Northway, I did notice a refinement in the reduction of file size by arranging the indexed colour palette, possibly darkest/lightest or most common colours or some other random method?
Not sure if it was IFF or gif/png but had some success, this was back in the day of dialup modems and end of Amiga Classics era.
A quick test for example of exporting a png-8 file via photoshop with no distinguishable differences in colours of some sprites has a file size changing from 10.03K and 10.13K file just by changing from either "Selective" or "adaptive" palettes.
The test file "Rygar" sprites has 20 colour and I was saving as a 32 png file.
I hope this is some use, if I find something else will add more detail, but back in the day when I descovered the reduction I was very surprised.
Jpg files, only success was using some extra tools with masking but was time consuming selecting which areas I wanted more compressed (usually flat colours)