Hank, I believe the issue related to proper album art is entirely dependent upon accuracy of the tagging in your physical USB track and how well that then matches to whatever semi-crowd-sourced WWW site Tesla is using -- generally Gracenotes or MusicBrainz from my various auto mfgr experience. My former Lexus, MBZ, and BMW that did the lookup, also showed the oddest art from time-to-time from radio, SiriusXM, and even my own tracks that didn't at the time have their own imbedded art. IMHO there isn't a lot that the auto mfgr itself can do to improve the art you see.
There is nothing you (or Tesla) can do about radio station lookups. You can try to improve your USB situation by perhaps changing track tagging, but that is really a shot in the dark since you don't know what will work via the WWW art sources Tesla is being used. As suggested though, adding physical album art to your USB tracks became the best choice a number of months ago when Tesla started supporting it, that will work
most** of the time. I suggest from my own testing you add "front cover" album art as JPG and only 300x300 pixels to maximize compatibility and minimize limited CID memory usage. The size can be larger (I've tested up through 1500x1500), but we don't know what that really does with other MP memory constraint issues I've discussed ad-infinatum in the Comprehensive USB Bugs thread.
** Compounding this on our MS for the last several months is the additional challenge whereby MP tends to cache album art even across CID reboots. So, just beware that if you remove your USB device and only add album art to a track without changing anything else, MP may still display the original art (the downloaded one in your case) until some uncontrollable-by-the-user magic happens inside MP... i.e. there are no guarantees how or when to get the new art to now display unless you change the directory structure and/or file naming associated with the changed tracks to trick the USB rescan process to look inside your track and pull out any new tag data including artwork.