This is from the Help section - give it a go:
Sync troubleshooting for cover images
If you're missing cover images after a sync or a restore from iTunes, you can force the program to re-sync all cover images by holding down the Option key on your keyboard and tapping the 'Sync' button.
Should you be using Pocketpedia as a standalone app and thus cannot do a clean sync from the Mac, you can re-download all the cover images from Doghouse by bringing up any large cover image in Pocketpedia (tap the image) and then hold two fingers on the large cover for 2 seconds to bring up an option to redownload all cover images from the Doghouse.
Cheers