FYI something else that might be worth looking into (which I happened to implement for iXForms) is allow clickable URL links in question hints. eg
<input bind="hintlink"> <label>Hint with link</label> <hint>For more information see <a href="http://en.wikipedia.org/wiki/XForms">Wikipedia</a></hint> </input>
Basically it allows you to hook in extensive additional question-specific background information/documentation via the hint, eg you could have a hint with "Tree Height Measurement" link to your image at https://opendatakit-a3b1.kxcdn.com/uploads/default/original/2X/a/aa9e2463cd03684f6d956c3c7393130eeba078bc.png
Although I'm linking to an actual web page above, with a bit of cleverness (and appropriate URL) you could easily have the link refer to an on-device media file, eg image, movie, HTML document, PDF, etc that is associated with the form.