Had the same problem... I think I solved that by removing the SD card from the player, removing everything from the internal memory, and turn on the player so it would rescan and destroy its current database (and it takes time, it's not immediate). Then you reinstall the files in the internal memory, put in the the SD, wait patiently, and after the scan the database should be correct. If not, you could also add a factory reset in there, just to be on the safe side.
Unfortunately, that I know of, there is no command to FORCE a rescan of the internal memory, the player does that only if it detects a change in the actual files (not in their tags)... this might mean that you could also simply take out the files whose album art you want updated, have the DX50 realize they are not there by turning it on and wait for the media scanning, then put them back on - it might work, but I did not try that (and unless there's just a few files, it might take more time to select the files to take out and put back in than simply replace everything
) ...
A command to FORCE a rescan of the internal memory would be very nice. As would one to force a scan of anything connected through the OTG (right now, the scan/browse option only applies to the microSD card, I think).