With all versions of MC completely closed, right click on Finder in the dock, Go to Folder... and put this in... ~/Library and press OK. Then go into the Application Support folder, then J River and then into Media Center 24 and you'll see the Thumbnails folder. Right click on it, copy it, go back to the J River folder and go into the Media Center 25 folder. Then either move the MC25 Thumbnails folder to the desktop or delete it, right click in the folder and paste (pasting over the MC24 thumbnails folder) then open MC25 and hopefully thumbnails migrated over.