Image link argument

If given, make clicking an image go to the specified URL rather than the image's URL as is the default.

By default, clicking on a rendered image links to the URL of the image itself. E.g. clicking:

would open Tank_man_standing_in_front_of_some_tanks.jpg as produces img surrounded by something like a href="Tank_man_standing_in_front_of_some_tanks.jpg".

If insetad we want the image to point to a custom URL, e.g. ourbigbook.com we could instead write:

and now clicking the image leads to ourbigbook.com instead.