The way I see it this is very close to Skittlish Dreams (http://www.dreamwidth.org/customize/advanced/layersource?id=83797&fmt=html) where I think Fu had to create an option to turn off the images in themes which didn't have them. If I want to create a theme for Boxed In but don't want to make images for it for whatever reason or don't want to use the existing ones it seems to me I should be able to and the layout should look fine without them (and no the CSS doesn't have to go into the themes but, if you create a prop letting your turn the images on/off, you can a variable dependent on this prop which would insert the necessary CSS, the way we often do with icon positioning). This is why I think everything should go into the themes, credit included, but I see Denise and Fu think otherwise.

And would it be good for me to upload the patch to bugzilla as it is so far so people can see what I've done and can make specific comments?

Yep!