You can change your stamp or postmark by viewing the front of the envelope on the Customize page, and clicking on either the elements. A menu will appear on the left (with an option to toggle between stamp and postmark options). Click on the design you want to use, and it will appear on the Envelope.
For stamps, your free options will appear grouped together at the top of the menu and Premium options will appear below. At this time, we do not have the option to upload your image as a stamp.
Postmarks are free to change to any of the options.