Magnatune now automatically detects retina display devices (i.e., iPad, HD Android, and 4k monitors) and swaps in album art and artist photos at twice the resolution in those cases.
I find the difference to be dramatic, especially with thumbnails of album art, with the text now suddenly being legible.
As indicated in another blog post, retina images are sent to you in WebP format, if you're running a browser compatible with that image format. In that case, retina images are actually 25% smaller than their jpeg non-retina equivalents!
If you're curious as how how this all looks, and you have a non-retina display, try looking at these two images: