I thought I'd share this method since it isn't mentioned in the FAQ topic on album art. You can add album art to an entire album without having to do it one song at a time.

In iriver plus 3:

- Highlight all songs in an album
- Right click and choose "Tag Batch Modify"
- Check "AlbumArt Batch Modify"
- Click on icon with blue plus sign
- Choose album art
- Click "Apply"

iriver plus 3 then applies the album art to all selected songs