I would create a subform with the "Image Field". Set the presence of that subform to hidden. Add a Checkbox to your form, "add image". When the check box is selected, it will display the subform where users can add an image.
Use this code on the checkbox - Change Event - JavaScript
if
(this.rawValue==1)
{subformName.presence="visible"}
else
{subformName.presence="hidden"}
Use the same code in the Inialized Event. This will save the state of the form after they fill it out and save it.