HDX and album art

After moving from HDX with internal HDs to HDX SSD all the music on my asset NAS, which has been downloaded from different sites (Naim, Linn, HD-Tracks, Highresaudio) the album art I had filled in by hand is gone and when I try to add it manually nothing happens, and all those albums have a hand under their logo

What does this mean and how can I add the album art again?