FLAC Album Art Discrepancy (iTunes)

I have had a few FLAC encoded albums in my itunes library for a while. I had just assumed that they were incompatible with attached album art. During a recent iTunes upgrade I noticed that quite a few of my regular mp3 album had been stripped of artwork. That led me to try the Get Album Artwork function. I was suprised to find that it worked and replaced all of the album artwork, and not just for the regular mp3/aac files. There was now album artwork for my FLAC albums. I first thought that this new version of iTunes allowed me to attach art to my FLAC files. I quickly realized this was an incorrect assumption when I tried to change artwork for Electric Ladyland from the censored US version that was fetched by the GAA function to a high resolution copy of the original UK artwork. When I selected Get Info for the album the artwork section was accessable (not darkened as it was before) and I pasted the new artwork in. Immediately after that I noticed that Electric Ladyland was once again showing up as a question mark. When I went back into Get Info for the album the artwork heading was blacked out and inaccessable. I had to run the Get Album Artwork function again to get the US cover art back.

This leads to the question. Is there some workaround that would allow me to attach artwork to FLAC files in iTunes? The images automatically assigned by iTunes are lacking in many aspects, from bad color correction and contrast to poor cropping and low resolution. It is better than having nothing appear but I would like to be able to have the high resolution, accurate album art that are attached to all the other non-Flac files in iTunes.
iTunes 11.0.4, Mac OS X (10.6.8)