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Do the audio files themselves perhaps contain album art?
I don't think so. Plain common MP3 ones.
Strange. I used the middle mouse click, advised by Barna and raina.
First, the square front moved to the left of the area, but with still an empty right part :

(Comment on this : isn't there a kb shortcut alternative for this middle mouseclick function ?
I noticed, that if the click is unintentionally done with some move, the next of the visuals in the list is called.
Quite a clumsy control I must say.)
So after a second middle click I get the menu:

Select Cover (2) ? Why ? There's only 1 cover 'inner_mounting_flame.jpg'.
But 'Cover (front) (ID3)' seems to be default.
So maybe
Dotpitch is right, that the plugin first scans for album art among the ID3 tags.
But if there isn't any embedded art among the Mp3 files, why pick a square of an existing bitmap ?
So I checked the second option with the filename, and then it was ok.
The right part with the text was shown as well, as intended. So the bitmap as it should be.
To re-check things, I did the first album 'Spectrum' again using the middle mouse-click in the frame:

No 'Select Cover (2)' there, just the name of the present bitmap 'spectrum_front'.
So maybe it seems to be caused by a specific MP3 detection script, because the files of 'Spectrum' are OGGs.
In order to check this behaviour further, I converted the Mahavishnu files from MP3 to OGG in the same dir with the cover bitmap, and made a new playlist.
All went well. Even when playing a single file, the wide cover art was correctly displayed.