When Picard embeds artwork from the MusicBrainz database it incorrectly calculates the "artwork_size" metatag value. I am aware of this because I was attempting to embed artwork for all of my mp3 files for display on the Bluesound App for iOS which is the control app for the Bluesound Node music streaming hardware. The artwork would not display on that app, which caused me to raise a support ticket with Bluesound. They responded as follows:

"Our Bluesound Lab has found what was going on with your Brain Eno file. The simple explanation is that the Metadata tag that describes the size of the artwork is wrong. 'artwork_size' should be 461658 bytes, but the file itself says that it is 42 bytes."