Hi - I'm new here, so sorry if I might not know all the rules.
I saw that nobody answered your question: I had the same problems. (Also using MAC).
I am also using a NAIM Superunity Streamer and this is also picky with regard to covers - therefore I am not completely sure which of the following points apply to the NAIM only and which to the AK100. I am always doing all of it since I want covers on both.
The AK 100 needs embedded art as far as I know.
1) When ripping I use XLD (Freeware) and add the cover art there. (If it is not found automatically)
2) For existing FLAC files which do not show the cover art I upload the files into MusicBrainz Piccard (also Freeware). You can also add the cover art here and then safe the files again.
From here on I am not absolutely sure which point could be left out for the AK 100
3) I still copy the cover in the folder where the music files are. The file has to be named "cover.jpg" - please be sure to name/rename the extension *.jpg - the file extension *.jpeg that MAC is using will not work!
4) The covers may not exceed 250 x 250 in size. You can resize existing coverart with Drag-N-Scale (Shareware - it costs not much but does what it has to very well) (I am doing the resizing BEFORE step 2)