Bluesky

Copy the link of the image. You see the bit at the end of the url that says ?format=webp? Change that to ?format=png.

Lemmy often doesn't show images in original quality unless specifically requested to.

Edit: which is fair, because the lossy webp is 51 kb vs 513 for the png. Compressed for longer, it could be a 265 kb lossless jxl though. Once Mozilla and Google finally add support (which is actually happening now!). It could also be a 322 kb lossless avif. All of these aren't max effort, just the effort that takes about 6 seconds on Image Toolbox on my phone

Lossily, avif > webp > mozjpeg > jxl > jpegli for this image, although I think this is just because jxl and jpegli use the same perpetual tuning method which must not favor dark areas. Which might be good for most images but certainly is terrible for this one. It certainly is much better at the bright areas. Mozjpeg vs jxl -> lossless webp (equivalent compressed size)

Note that all of the lossless formats would have been much smaller if the original screenshot in the mastodon post was lossless