In Gromit, audio files (MP3 format only) may be added to an album project in the same way as image files or video files. A generic image is provided for audio files, to serve both as a thumbnail image on the index page and as an image to be displayed on the slide page for the audio file.
It is possible to provide your own representing image for the audio file. Choose a full-sized image (a regular JPG file, and not just a small thumbnail), change its extension to .thm, and give it the same name as your audio file. For example, for an audio file called mydogsings.mp3, the image file should be named mydogsings.thm. This file should then be added to the album project at the same time the audio file is added to the project. This image will then be used both for the thumbnail and for the slide page, where it will be displayed while the audio is playing.