When attempting to download cover art for certain releases (examples below), Picard will show errors in the information dialog (Context menu "Info...") if a release has images that are GIFs. This seems to happen for both animated and static images.

Other image formats are downloaded as expected.

I have Picard set to download all cover art types.

Screenshots:

• Album Info dialog:
  
• Album list (note the count of 7 images):
  
• MusicBrainz data (8 images):
  

Releases with GIFs I encountered (excerpt):

• https://musicbrainz.org/release/36cf1c7c-88a9-4204-99bd-6ab2531171d3/cover-art
• https://musicbrainz.org/release/25dac7c3-4f4f-49bd-af82-13b4091edce9/cover-art
• https://musicbrainz.org/release/478cf157-ad1e-4a25-b500-a7117a9bc8ac/cover-art
• https://musicbrainz.org/release/292f1d2d-4814-32b1-af60-8c8ccb7b2f7d/cover-art
• https://musicbrainz.org/release/d6e602eb-97e5-42ca-919b-37c2ea510bad/cover-art
• https://musicbrainz.org/release/7020b316-5e18-4e75-b8bd-c0a339315108/cover-art
• https://musicbrainz.org/release/b48b8b50-05d7-4d52-863b-02d901112d7e/cover-art
• https://musicbrainz.org/release/005bc8e2-295a-4499-ad97-a1f83099cbe4/cover-art
• https://musicbrainz.org/release/4b6a00f7-f820-410c-8436-da7320fd13a3/cover-art

With that out of the way, thank you so much for this wonderful piece of software!