The built-in audio player has MPRIS2 integration, which also can display cover art. Currently this works by generating a temporary file and sharing the path to that file as a file:// URI.

When running Picard as a Snap this breaks, as the temp file path created inside the Snap sandbox is not valid outside.

One solution to this could be to utilize the built-in web server to serve the file. That way no local paths are involved.