Feerix,
Are you refering to when you use MC9 to automatically download images? Or associating cover art manually? Or KingSparta's awesome Lyrics/Chart/Cover Art plugin?
With the built-in MC9 "cover art" options, I've found that the "download images from the internet" option works pretty well about 50% of the time (the more well-known/popular an album/artist is, and how correct your tags are making a big difference, of course . . . I have a lot of indie/underground stuff, and tracks wihotu real album names, which generate odd results, as is expected), but I still have to fill in a lot manually (or correct various mistakes). Although it's a really exciting feature of MC9, the cover art function isn't perfect, certainly, but I don't think I've had the problem you're having. I'm looking forward to trying out the Cover Art Lookup version of King's Lyrics/Chart Finder (which was already a truly awesome plugin before), but I haven't had a chance yet .
My suggestion, for now:
To have different images for your different Robbie Williams tracks, simply download the images you want, then place them in the MC9 "cover art" folder (In the JRiver folder tree -- you can actually keep these images anywhere you want, but this is the default folder). From within MC9, you should be able to then associate the individual track with any images you want by browsing for the image. If you give each Robbie Williams' track a unique album name (say "Best of Robbie 1" and "Best of Robbie 2"), and then name the image files in the same way, "Robbie Williams - Best of Robbie 1.jpg" (and so on), then you should be able to just highlight the Robbie tracks in question and then use the cover art option to auto-update from your image files (which are connected to the audio tracks via "artist - album.jpg").
That's more or less how I've been doing it, depending on the circumstances. . . I'm still playing around with all the possibilities the cover-art functions offer. For the most part, though, I put all of my random/misc audio tracks by a particular artist into a folder called "Best of..." or "Rare tracks" or "Live" (under the cooresponding artists' folder tree) . . . that way I can just associate all of the tracks in a given folder to one image, which might not be as nice as having a different image for each track, but is much simpler, and has other organizational benefits to boot.