It can be challenging to find cover art for DVDs. Three sites seem to get just about everything for me.
Cdcovers.cc – Contains fan scanned DVD covers. Many are non-english and non-US and in a wide variety of resolutions.
Amazon.com – has small images and some have this zoom feature that makes them unobtainable and Flash-y.
Netflix – seems to have everything, but only in tiny 5 kb files. Last resort.
Just right click the image and save it. Don’t forget to crop it. Backsides from CDCovers and white edging from Amazon.com are annoying and unnecessary. Macs make this exceedingly easy with Preview.