Hi Joe,
I just looked at the code and I don't remember doing this so I think someone else worked on this at some point, but it looks like you can use "skinbase_" as a prefix to an image located in the skin folder so if you had an image named search.png you would put skinbase_search.png and it will replace the skinbase_ part with the base url to the skin folder.
Alternatively, you can put any path there in the format ~/path/to/yourimage.png
Hope it helps,
Joe