Description of issue:
When I set Picard to download cover images to embed in tags, it works fine at every resolution except 1200 - which is unfortunately my desired resolution.

Settings are found in Options -> Cover Art -> Cover Art Archive: "Only use images of the following size":
Options are (in pixel widths): 250, 500 (*default), 1200, or Full Size.

When I have the setting on '1200', the downloaded images are always 500.
I looked through the Picard debug logs and I think I found the issue - the Archive.org JSON service is not returning the 1200px jpeg's as an option (even though it has them).

How to repeat:
1) Check Picard cover art setting is set at 1200.
2) Load a set of files for a release into Picard. Ensure it has a Front cover scan on MusicBrainz exceeding 1200x1200 pixel. For example the release I have just been testing is this one by Isis:
https://musicbrainz.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/cover-art
[has 1200px art, and full size is 1500px × 1500px]
3) Scan the files to match the files to the correct release with AcoustID/etc & allow Picard to download the release metadata and cover image.
4) Check the downloaded cover art in Picard by selecting the album with left-click, then in bottom right where the album art is usually displayed in Picard, select 'Show more details' button. It should show the art downloaded is 500 pixels wide.

Log data (emphesis added):
D: 19:29:40,715 webservice._handle_reply:559: Received reply for https://ia600306.us.archive.org/16/items/mbid-0ba88c6e-cc27-4d6b-bf98-638464ec069e/index.json -> HTTP 200 (OK)  (CACHED)
D: 19:29:40,716 webservice._handle_reply:572: Response received: {'images': [{'types': ['Front'], 'front': True, 'back': False, 'edit': 30522707, 'image': 'http://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524.jpg', 'comment': '', 'approved': True, 'id': '9123370524', 'thumbnails': {'large': 'http://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524-500.jpg', 'small': 'http://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524-250.jpg'}}], 'release': 'http://musicbrainz.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e'}
D: 19:29:40,716 coverart/providers/caa._caa_json_downloaded:293: CAA types: included: {'front'}, excluded: {'raw/unedited', 'matrix/runout', 'watermark'}
D: 19:29:40,716 coverart/providers/caa._caa_json_downloaded:313: CAA image accepted: http://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524.jpg  ['front']
D: 19:29:40,716 coverart.queue_put:205: Queuing cover art image CaaCoverArtImage(url='http://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524-500.jpg', types=['front'], support_types=True, support_multi_types=True, is_front=True)

NB: The 1200 link exists at the same URL (with '-1200' appended) and loads fine, it's just not offered in the list of available covers by Archive.org's JSON webservice:
'https://coverartarchive.org/release/0ba88c6e-cc27-4d6b-bf98-638464ec069e/9123370524-1200.jpg'

Thanks for your time reading that, and your help. Cheers