How do the Udacity programers produce those questions, with checkboxes on top of images?
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In [here](http://www.udacity.com/view#Course/st101/CourseRev/1/Unit/6001/Nugget/119001) and [here](http://www.udacity.com/view#Course/cs253/CourseRev/apr2012/Unit/20006/Nugget/73002), there are some examples of the quiz questions that are used in Udacity. 

How can I achieve such an effect ? (i.e. : how can I build a page where I have an image, and then some form elements over that image ?)

Maybe I could use a table with the image as background. Would that work ? Would it be reliable across browsers ?