I might have missed your point, but in the hope that this might help, please ignore if I am stating what you already know. If you want to get cover art from the web go to Admin/Files&Media/Media Files Sync. On the LHS in the directory structure navigate to the location of your file. Click on the location and your file should appear on the RHS under "Files on disk". Click the name of the file. Click the "Check Amazon" button. Click "Please select" and choose "Title" from the drop down box. Click "Get Amazon Attributes & Cover Art" watch in the lower status bar for "Waiting for dcerouter" to finish . The RHS should now be populated with metadata and cover art of items matching your search criteria. If you find something suitable click its radio button and scroll down to the bottom of the page where the "Save" button can be selected. If nothing suitable has been found use the "Back to file" button and try another keyword search.
If you want to add you own cover art from a photo or scanned item the jpg image may have to be small. Mine are generally less than 350x500pxls. Use Linux command lines to place the picture into a directory then navigate back to your avi file as described earlier but instead of using the Amazon buttons scroll down the page to the Add Picture area and use the browse button to locate where you have placed your picture. Click the "Add Picture" button. Save, resynchronise, quick reload router and you should see your cover art. I have had to power down and restart on occasions to see the newly added cover art.