Hi Greg,
You can use the signature widget for this. I'm attaching a form that allows you to do that. Unfortunately the current KoBoForm doesn't yet give you the option to create this kind of question in the user interface, but it's easy to edit the XML file to include this part.
If you open the attached XML in an XML editor such as Notepad++, you will see
appearance="signature" that was added on line 31. (or check it here:
http://pastie.org/9283837)
All you need to do then is first create your form with an image question and then add appearance="signature". Instead of being shown the camera the user is able to add their signature on a line.
Hope this helps!
Best,
Tino