When converting a wave file to a FLAC file using the flac command, an album cover can also be added by using the –picture argument with an input image file.
$ flac -0 ... --picture="3|image/jpeg|||cover.jpg" ... filename.wav
where 3 means that the picture is a front cover, cover.jpg is the cover image file, and filename.wav is the name of the input wave file.
Alternatively, metaflac can be used to add an album cover to a FLAC file.
$ metaflac --import-picture-from="3|image/jpeg|||cover.jpg" filename.flac
where the format of the –import-picture-from argument follows the –picture argument of the flac command, and filename.flac is the name of the FLAC file to be processed.
Note: flac and metaflac may not be present in a default installation. It can be installed through the package manager.