Almost without exception, my album art on my Mac looks great. There is the odd one or two albums that I cannot find a decent image for, and those a re a lot more pixelated.
That said, they ALL look pretty lousy on the F90. A lot worse than they do at what ever size I could see them on the Mac itself.
I am of the opinion that it something to do with the way the software of the F90 is "reading" those files. The reason I say this is because the image I used for my Splashscreen looks great. All the art files for the iPod, not anywhere near as good.
Do I wish they looked better? Yep. I just don't have the knowledge to fix it at this point. Something is better than nothing at this point, I guess.
My OEM head unit sure didn't show album art! 
Cheers,
Kev