Random House has a bookseller services site where they make their book covers available publicly. This site is slated to close soon.
See:
http://smartpublic.randomhouse.com
They claim that the files will still be accessible but I haven't seen where. The covers on this site are high res, usually 2400px in height for relatively recent books. The covers on the rest of their site are much smaller.
If you've downloaded Random House ebooks before, you know that many of them come with default covers, displaying only basic info. Covers available elsewhere are inconsistent in terms of size and quality, so a site like this is very convenient.
You may want to grab some covers before it all goes away.
Note that their search facility is fairly rudimentary. It requires exact matches. So "The Bookname" is different from "Bookname" or "Bookname: A Novel" and so forth (In the latter case you can use wildcards).
If you can't get a title match, look on Amazon for an ISBN and search on that (no dashes or spaces in ISBN). Once you see the official title, search again with the title.
An ISBN search will only return one result, but a title search will return all matching variations, so you'll see all the different cover sizes and versions (hardcover, paperback, Canadian).
There are different Author fields but the only one that I've been able to get to work consistently is the "Author Display" field at the bottom of the screen (First name Last name, no commas).
See the attachments.