[PlasmaKrab]

I agree, getting a lightweight front pic and a link to an archive may be the best solution, though more complicated.
PNG pics I have tested tend to stick to the original colors but don't lose much weight. Anyhow, retroconversion in itself is a moot point since even the Windows image browser can handle PNGs. Now if there has been the slightest change in the image, the end user will need to reindex it to the palette, which 1) isn't something everyone will spend time to do and 2) takes a large bit off the point of having icons delivered ready to use when in fact they aren't.
It all depends on the storage space and working time you are ready to devote to this.

BTW, do you store and sort LBMs as well?