Support GIFs

[Commenter25]

Currently, GIFs are completely unsupported, whether static or animated.

Static GIFs can become PNGs. You can convert to PNG-8 for increased size reduction, with no loss since it's the same restriction as GIF. This also prevents confusion from websites which assume GIFs are always animated. There is precedent to this, as this is what PNGGauntlet does.

Animated GIFs can be compressed directly using Gifsicle, both losslessly and with the --lossy option. Alternatively, they could become APNGs, which are backwards compatible by displaying the first frame statically. However, while software support is common, websites tend to only interpret GIFs properly and not APNG.