What's the best resolution for album art?

I have encountered some 500x500 album art images that don't fill the album art box, and some 200x200 album art image that do completely fill the box. So I assume that there's more to importing album art than just pixel counts.

That leaves pixel density, or resolution. What should the proper resolution be for a 500x500 pixel album art image? Anything else I'm missing?

Thanks. WeedMonkey