You probably need to change the moviejukebox.properties file to include "filename.nfo.skipUrl=false" as described here...
http://www.mede8erforum.com/index.php/topic,8279.0.htmlIf you do not add this parameter, then YAMJ (not Y2M) will ignore the <thumb> entry in the NFO file (though it will still pull the default thumb image, just not any you specified in the NFO). You also need to make sure the <thumb> entry points to a poster with the correct aspect ratio...It should be 50% taller than it is wide, like 1000x1500.
You might also try putting each episode in its own folder when running YAMJ, then renaming the about.jpg to the same name as the episode (but with a jpg extension instead obviously) before copying the files back to the main folder.