[feature] Encode thumbnails into format that keeps transparency

[tsmethurst]

Currently I think we encode thumbnails to .jpg format, which is fine for static images that don't have an alpha/transparency layer, but is a bit of a shame for images that use transparency.

We might consider encoding (gif, webp, and png) image thumbnails to png instead, to try to keep transparency, but we'll have to investigate the best way of doing this.

Re: #3040