unfortunately it is not related to files which are currently "in use".
Every now and then I load a whole bunch of albums into MC and do the tagging without listening to music or doing something else.
All of the files are mp3 with ID 2.3 tags from various sources, some bought online, but most of them my own ripped CDs, converted from wav to mp3 by MC itself. Of course those don't have a cover image when "native".
Some images come through MCs search feature, but usually I prefer to use the covers I have searched for elsewhere or scanned myself and saved in a different database. When needed, I copy the image to the clipboard and use the paste feature which comes with MC.
Interestingly, I only have tagging issues with cover art images, never with changing other tags in a batch procedure.
And it should have nothing to do with image size, because (as mentioned above) I limit the size to a maximum of 500 pixel in any dimension. So all of the images are about the same size, some even smaller.