In her article, Would You Sign My Kindle, Stephanie Rosenbloom shares how a digital autograph signing would work:
... a reader poses with the author for a photograph, which can be taken with an iPad camera or an external camera. The image immediately appears on the author’s iPad (if it’s shot with an external camera, it’s sent to the iPad via Bluetooth). Then the author uses a stylus to scrawl a digital message below the photo. When finished, the author taps a button on the iPad that sends the fan an e-mail with a link to the image, which can then be downloaded into the eBook.
