Itunes album covers I import are fuzzy, jpeg-quality.

I designed this great, high-quality, crystal-clear album cover at 600x600 pixels for an album of music I made (.png format). I got the songs, added the album cover for those songs, and synced it to my ipod. It looks great on itunes, but on my ipod, it's a bit fuzzy or blurry, and the picture has those jpeg-type markings on it. I tried changing the size to 900x900 (still keeping the entire image sharp) and even changing the file type to .jpeg, but alas, no increase in quality.

I've imported album covers before without this problem. Any ideas as to why this is happening?

Thanks in advance. 🙂