Find the coverart you want online and copy the URL.
Right click the particle and go to Cover Art > Paste from Clipboard.
I know how to assign cover art, the problem is that the particles all still appear to be sharing the same cover art. So if a DVD has 2 different movies or shows on it, the last particle to have cover art assigned replaces the cover art for the other particles.
Two examples would be: How the Grinch Stole Christmas/Horton hears a Who. The Steve Martin collection with 'The Lonely Guy' and 'Dead Men Don't Wear Plaid' on the same disc.
I did have the separate titles ripped to separate MKV files, but MC doesn't handle MPEG2 in an MKV container properly on all of my systems. Trying to use full DVD copies and particles (since this does not seem to be a problem for MC on any of my systems), but I can't assign unique cover art for each title.