General note: if you want high-resolution JPEGs but upload higher-resolution non-JPEGs, you can download the 1200px “thumbnail” generated by IA. It should be available for all new/recent uploads and some(/most?) older ones too. There is a userscript that exposes this on the MusicBrainz interface until CAA-88 is resolved:
It should be easy to add this as an additional option in Picard for yourself as well, but it probably won’t be upstreamed in Picard until CAA-88 is resolved either.
(I also upload (probably too) huge PNGs to CAA, which are slow to load on my connection when I need to look at them again for getting credits off of them for entering… So I wrote this userscript so I could see a high enough resolution image that I can read the small print of most booklets without waiting a month for each piece of “original” CA to be loaded…)
Low, 300× resolution artwork that is still accurate is better than not having any at all. Yes, high resolution scans are preferable, but 300²px ones will do until someone adds something better.