iTunes handles artwork a little strange.
Most of it is not stored inside the music file but only resides in the iTunes "database".
It is possible however to store artwork inside the files, too.
Here you can find a lot of scripts that are useful for iTunes.
Under "Artwork" there are scripts that embed covers, so JRiver MC can see them.
One trick:
iTunes imports covers automatically - but only in 300x300 resolution.
To get really sharp covers (600x600) with good colors in iTunes, you first have to erase the "automatic" covers and then let iTunes search for covers for the (selected) files again.
This will result in 600x600 covers in high quality.
Afterwards you have to embed the new covers (by using the script) into the file(s).
Even iTunes 11 doesn't do that automatically as my tests have shown.
Thats a lot of work, but in the end you have beautiful covers in bright colors.