I found that for a release I am adding, the publisher actually gives a high resolution TIFF file of the cover. See:
Is TIFF support in the works, or what else is there to do if I want to add the original file to the cover art archive?
There's a ticket for .tiff support.
Oh, also, the only place I was ablle to find a list of supported formats is here. Hmm. There's probably a more sensible place to put that list.
Just convert it to PNG. The file size will be much smaller and you won't lose any quality.