You missed this part.
Code:
<img onClick="BildAnpassen(this)" onLoad="BildAnpassen(this)" src="/bild.png" title="Klicken zum Vergrößern/Verkleinern">
First I would like to ask the others on this board if there isn't a better zoom/lightbox image script (e.g. jquery plugin), which is suited to surveys? What about
dimsemenov.com/plugins/magnific-popup/
?
Problem is finding an maintained plugin.
Not sure if a 100% errorprone solution is possible, but you might add a hidden question and force an answer. When the image is clicked a certain value is written into that hidden question.
As long as the hidden question is not answered the image has to be clicked.
Since I am not the biggest javascript fan I cannot provide you a readymade solution.
Perhaps my silly lines will provoke a more sophisticated answer from the household user names of this board.