I found the below optional support in one of the mt-daapd.conf, may I ask which version of mt-daapd support it because I tried on Firefly snv-1696 but folder.jpg doesn’t show up when playing the songs…
#
# art_filename (optional)
#
# There is experimental support thanks to Hiren Joshi
# (hirenj[at]mooh.org) for dynamically adding art to the id3v2
# header as it is streamed (!!). If you were using a music system
# like zina or andromeda, for example, with cover art called
# “_folderOpenImage.jpg”, you could use the parameter
# art_file _folderOpenImage.jpg and if the file _folderOpenImage.jpg
# was located in the same folder as the .mp3 file, it would appear
# in iTunes. Cool, eh?
#
art_filename = folder.jpg