Well, I just did it in Photoshop when necc. I generally just use Google Images to find the appropriate cover in the right size. I only have to resize on rare occasion. You could easily set up an automated workflow in Photoshop if you have access to it. If not, I think you could probably just use Automator if you have OSX.

EDIT: Yeah, I just played around with Automator a little. Just open it up and search for "scale images". Again, this assumes you're on a Mac (which for some reason I thought you were...)