Add a note above the form to forewarn users that if they can't view
images, then they won't be able to use this mechanism. In the words of
the user who suggested this:

I always see and work with the page from the top down, so I
filled in this form before realising that I was completely
incapable of getting it submitted which was obviously a tad
annoying.

Discussed with: Sabahattin Gucukoglu <mail@sabahattin-gucukoglu.com>