Do you have an MC10 library backup file? You can restore an MC10 library backup file in MC12. A library restore restores everything that you had in the old library, including the cover art links. Of course you need to have the actual cover art files in the same place where you had them before. If I recall correctly MC10 saved them in a subfolder in the [MC program path]\Data\ folder (... by default). In MC12 you can find the restore tool here: File > Libary > Restore Library...
If you don't have a library backup file, but you happen to have the MC10 data folder intact you can create a new MC12 library there (= in the subfolder where the .jmd files are). MC12 will automatically convert the old MC10 library to the MC12 format. (Back up the old data folder first.)
If you have MC10 still installed you can create a new library backup file and restore it in MC12.
Edit: fixed a couple of errors & typos