In my experience most images already have a 1200px version available. I even made a userscript to expose these on MusicBrainz.org. See this for link to userscript and additional comments on the matter:
Picard probably won’t get the 1200 option until the backlog of images have had a 1200 version rendered, but for browsing MusicBrainz at least, my userscript has made it a lot more pleasant for me.
