How do you add an electronic signature or image to the form?

Method 1 (automatic)   
First, you will need to assign the image you plan to use as a signature using the File - Preferences command. Once the image has been assigned, each time you move to a field called "Signature" on any form the "Sign Form" button will appear on the top ribbon. Simply click the button and the assigned signature will be inserted properly in that field.
Make sure the image is cropped (no white-space) as close to the edge of the actual signature as possible. This will result in a more realistic appearance on the form.

Good. Very little white-space.

Bad. Too much white-space.
A sample image of a scanned signature (SampleSignature.bmp) should have been included with the installation of GFS. If not, you can download one from here:
Method 2 (manual)