Looks like a bug: once artwork filed is added to now playing list in album & tracks view, no way to remove the field (it make sense in the main panel, but not in the now playing list)
For now, the only way to remove the field seems to edit MusicBeeSettings.ini file.
Locate the following line:
<FormNowPlayingSidebarFieldsAlbTr>
Then you will find something like this
<FormNowPlayingSidebarFieldsAlbTr>
<MediaField>
<Code>40</Code>
<Width>131</Width>
</MediaField>
Remove from <MediaField> to </MediaField> (artwork field)
Save and relaunch MB.