Just to be clear about what you have been doing.
So have you created your own custom skin, and does it contain image elements for UnknownArtist and NoArtwork?
If so, do these images show up properly in other views?
You also might follow up on Mayibongwe's suggestion to test this using some other (more recent than beetunes) skins.
It won't modify any of your settings, and it could reveal more about the actual issue.
edit
If you are going to do some more testing with other skins, I am sure that
Elemental has correctly functional images for those, and that
Sample Skin does not have them at all.
These two might be helpful to see what's what.