Noticed something with Bluesky's image URLs. There's a modifier at the end that lets you choose between whether you want to see a JPEG/JPG version or a PNG version:

Modifying the URL to end in @png instead of @jpeg results in the downloaded image being of higher quality. Here's a post and two downloaded images as an example:

The post: https://bsky.app/profile/fruttymoment.bsky.social/post/3lcdqq3x47s2h

Then downloading after clicking on the image to view it in full as opposed to the preview:

Then modifying the URL's ending parameter to show a PNG instead of a JPEG:

If you zoom in on both images you can notice that the PNG indeed doesn't include the compression artifacts that the JPEG has (although they're relatively faint here since the image is so simple). I tested it with an image I presumed to be already compressed and Bluesky seems to recompress JPEGs since the PNG version also contains some compression artifacts but not as much as the JPEG version. Although technically I can't be exactly sure if the image I used as an example was uploaded originally as a JPEG as a PNG or just as a JPEG, I'd assume it'd be the latter since the former would take considerably more effort. (Example image here, feel free to test it out and see for yourself: https://bsky.app/profile/oofmasterxd.bsky.social/post/3lchnegnrfj2b)

I'd like to request that gallery-dl is modified to have an option to download in either JPEG or PNG quality, and additionally that this option is set to PNG by default (although I'm still fine with downloads remaining in JPEG by default, just that the option is there to download in PNGs).