To enable this feature in the form, go to:
Attachment field
Upload settings section
Select Hand Signature from Field Mode.
When the field is set to "Hand Signature" mode, it will automatically be set to only accept one file (the signature), and to use the "Contain" Thumbnail mode. These options can, of course, be adjusted to your preference.
If you want the signature to be mandatory, you must set the attachment field as Required.