In mp3tag, there are multiple cover types listed (see attachment):
These are the allowed image type labels. To include an image label for an added image in Mp3tag,
RMB on the image > Set cover type (as you have shown) and choose one of these labels for that particular image.
Example: For an album in Mp3tag, select all tracks. First add the front cover image and then RMB on it and select cover type "Front Cover". Next add the back cover image and select "Back Cover". Add a leaflet image and select "Leaflet", etc. Then save all. This embeds the images and their labels in each selected flac file, sequentially in the order you added them. BTW, while you can remove an image, Mp3tag has no way to go back and "insert" an image into the list (something I requested long ago, but has never been implemented). The addition only happens sequentially from start to finish.
For efficiency, RMB on any track in MC and select
Send To > Send To (external) > Add/edit programs find and add Mp3tag.exe. Thereafter Mp3tag appears directly in MC's RMB track menu, usually in the top section showing my recently used commands. Invoking it automatically opens the entire album folder in Mp3tag.
JRiver MC ignores these labels and multiple images. It displays only the first embedded image as cover art, no matter what its label is. So beyond the front cover I go through this process for "archival" purposes, hoping that some day MC or some other program will be able to display multiple labeled images stored per flac standards. My starting point is always the images stored in the album directory alongside the audio files.