Hi pot.s...,
I think that the issue is as follows.
Although it is widely believed that the Brennan does not use tag data (that is non-music data that is stored inside each individual music track file), this is not entirely accurate.
The Brennan WebUI uses two sources for displaying cover art.
1. The file 'coverart.jpg' which is stored alongside the music tracks in the <album> folder. On the WebUI this is displayed in the right hand window, in the 'Album' field.
2. In the WebUI, In the top left hand window where the 'currently playing' track information is displayed, that image is stored in the tag data for that track.
The Brennan mobile app appears to use only the tag data, but I only ran a quick test on that element.
Regards,
Peter.