Imaging from UNC path works only in very limited environments - small or very localized ones. Highly distributed environments can't use that concept or the images would be pulled over a WAN link - not a good idea. It's a valid suggestion, and we have that and several other "similar" suggestions captured, but you're right, not implimented.
As for "can you change imagepath... Good question!! I don't know. Obviously not supported, but it seems to me that you should be able to do so frankly. The problem is I'm not sure how to edit that line, or, more importantly perhaps, how to make sure the path is ALWAYS the same, but the image file itself is the variable. In the case of the default, the imagename is both the path AND the image, so it could be tricky. Not to mention hidden from fiew.
Fascinating. Let us know if you find it! I might try to find out too. :D