After some research, I discovered that saving to PNG format would require the TPNGImage library, as well as an additional unit posted here. That's not too big a hurdle, but TPNGImage is not licensed under a standard open-source licence; it just has some simple terms and conditions. I'm not sure whether it's compatible or not with MPL 1.1, so I'm deferring the feature which would save thumbnails to PNG for a future version.