Right-click on the album, select the Artwork tab, then use any of the following methods:
- Click the Add Artwork button, browse to and select the appropriate image file
- Drag and drop the appropriate image file from Windows Explorer to the Artwork tab
- Copy the image to the Windows clipboard (either from a graphics application or a web browser), go to the Artwork tab and press Ctrl-V.
All three methods will work unless:
- The media files in your library are in WAV format, which does not supported embedded artwork images, or
- The media files in your library are read-only, or
- Windows permissions are preventing updates to your media files.
In the first case you'll need to convert the media to Apple Lossless (ALAC) or AIFF if you want to preserve audio quality, or to AAC/MP3 otherwise. Read-only and permissions issues can be fixed through standard Windows resources (right-click Properties in Windows Explorer).