Added image example to specific test page

And changed the documentation a little so that it isn't just an out of
context cut and paste of jo3w4rd's example.
Switch to using Sphinx "absolute" paths for images

The code was getting confused when the .rst file that wanted the image
was not at the top level of the Sphinx project.

Here we change to using the Sphinx "absolute" path concept but
prepending "/" to the relative path from the Sphinx top-level folder.
Without the "/" it thinks that the relative path is from the subfolder
containing the .rst file and so gets confused.