Normally, the images appear together with the list of albums or songs in the Source panel.
Please open iTunes and find one track that has an artwork. It must be displayed in iTunes right panel. Then in Disc Cover, import that track from iTunes and see if the image was imported too. If it wasn't, send this audio file to email@example.com
mentioning this forum. We'll see what's happening.
Also if importing an image from that test track is unsuccessful, save your console log and sent together with the audio file. Probably there are error messages.
To save the console log, open the Console application from the Applications/Utilities folder. Choose File > Open Console Log. Then choose File > Save a Copy As in the menu of Console to save the log.