I did some investigation in 2017 (https://community.mp3tag.de/t/anzeigen-von-bildinhalten-im-tag-panel/19557) and came to the conclusion that Mp3tag uses the following order:
a) folder.jpg
b) any image file with "folder" in the file name
c) cover.jpg
d) other image files in alphabetical order
So a workaround for you could be to place an additional folder.jpg or cover.jpg in the directory.