To address the original issue and save a lot of work in converting files from WAV to whatever:
I received an upgrade notice today. I am reluctant to upgrade. I did one upgrade and had to spend an inordinate amount of time reapplying my album art.
24.0.5 (3/13/2018)
8. Changed: When upgrading from MC23 to MC24, the images in the cover art folder will be copied over from MC23.
24.0.8 (3/29/2018)
2. Fixed: Copying Cover Art folder from MC23 to MC24 during upgrade installation did not work.
Are you running MC23 now? If so, and if you have left the Cover Art folder in the default location, the upgrade process to the current MC24.0.30 should find all your existing Cover Art and copy it to the new Cover Art location for MC24.
If you have changed the Cover Art folder location, as defined in "Options > File Location > Cover Art", and you have placed it in a non-version specific location such as "M:\Cover Art" or similar, then MC24 will just continue to use that location after the upgrade.
Even if all the above fail or do not apply, fixing Cover Art between versions is extremely easy and does not require an inordinate amount of time. No reapplying is required. Just copy the folder;
"C:\Users\[UserID]\AppData\Roaming\J River\Media Center 23\Cover Art\" to "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\Cover Art\".
EDIT: I see you are probably still on MC22 Resgostar. In that case, if the new functionality doesn't copy over the Cover Art folder for you, just copy the folder;
"C:\Users\[UserID]\AppData\Roaming\J River\Media Center 22\Cover Art\" to "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\Cover Art\".
The Wiki article is quite good, and worth a read:
https://wiki.jriver.com/index.php/Cover_Art