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.