Simply click the 'Add' button at the bottom of the Fields section. You will then be presented with a choice for where to add the image. This can be either the form description, a field's description or the form footer. Pick where you'd like to add an image and then select the image icon from the toolbar.
The image will be displayed with the field/description when the form is filled out:
When editing records or using Save & Continue mode, you can alternatively use a read-only attachment field to display an image from the record.