The iTunes store is a good source of high-quality album cover art. If you
search for the album on Google Images, then
visit the page and right-click on the cover image, you will get a 170 px
by 170 px image. Change the 170x170
in the URL to one of the following
values to get a higher resolution image:
64x64
100x100
170x170
340x340
600x600
1200x1200
1400x1400
Alternatively, use this handy webapp to query the iTunes search API and get to the source image directly.