Notably, the forum has PNGs (and SVGs) anyway in the form of emoticons (not just font-based emojis) and the actual topic icons. Seems like an oversight to me, given that PNGs have only been the standard web image for a quarter of a century.

Sure it's nothing to convert a PNG to a lossy old JPEG that has blurry speckles everywhere and is 6x the size unless you compress it to death for even less quality, or converting to a GIF that only allows 256 colours due to the format being last updated in 1989, but still...