I've searched high and low on my system for the image that is displayed in the Skype indicator but is nowhere to be found, even the Skype deb file doesn't have anything like it and no other Icon set changes the image.
Is it possible to? at all?
It seems to be hard-coded in the binary. As long as it is not an open source program, you are not allowed to change it. The discussion regarding this topic dates back to 2008!