The actual image files on the disk are separate from the more or less temporary thumbnails that MC creates for quick thumbnail display.
If you started with a copy of your MC11.1 library the link data should still be in the library. Check the image file field inside MC12. Do the filename links point to the folder that you used for the captured image files?
MC12 should automatically create new thumbnails from the linked image files. Unfortunately this does not work. I have reported about the bug in the 12.0.76 thread:
http://yabb.jriver.com/interact/index.php?topic=35893.msg244625#msg244625.
I tried to use the "drag and drop into the AW image display window" function, but it seems to be broken as well.
For now, you need to relink the image files with the Image menu tools (Right-click > Image > Remove Cover Art > Right-click > Image > Add From File).
You may want to wait for the fix if you have a lot of video files with external cover art.