They print the picture on rice paper then simply lay it on top of the icing. The dye is a slightly modified food coloring so the whole thing is edible.
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How is this possible? It's simple. The person wishing to have a photo cake made gives a favorite photograph to the baker. The baker will make the cake as usual, except that instead of embellishing the top of the cake with pink or blue flowers, he'll adorn it with an exact copy of the photograph.

To make the photograph for the photo cake, the baker will scan the provided photograph. Once the photograph is scanned, it's printed out on special edible "icing paper" made from rice, potatoes or cornstarch. The ink used to imprint the picture onto the paper is made from food coloring.

A rice or potato icing sheet might actually add an extra thin layer to the birthday cake. Though this won't take away from the overall look and taste of the photo cake, it is apparent where the icing sheet was placed. There are special icing sheets made from cornstarch, however, which are made to dissolve once they're placed on top of cake icing, leaving only the photograph behind.

If you're willing to invest in the equipment, you can also make a photo cake at home. The supplies necessary to make your photo cakes can be ordered over the Internet or from special mail order companies. Included in the photo cake-making kits are edible inks and frosting sheets. The supplies are expensive, though. One can expect to pay between two and three hundred dollars for one kit.

There's another consideration for those wishing to make a photo cake at home. While the kits are designed to be printed from your home printer, think carefully before you do this. If you are going to be running food through your printer, you'll need to first do a lot of scrubbing. The last thing you want is for your guests to ingest ink that was left behind on your printer. Once you're finished printing, you'll need to again clean your printer thoroughly. Unless you're planning on purchasing a separate printer, making a photo cake at home is not recommended.

--a supplier in India offers the following...

Photocake technology is already reaping riches around the world. Now scan your favorite photographs, Greetings, Sketches, Messages or Wishes and print it on our special edible Icing sheet also called Frosting sheet. It's just that simple. You need no special skills for photocakes.

A really fantastic opportunity to bakery's and cake shops to digitally decorate and add value through photocakes. With this technology, using photocake edible Icing sheets, photocake edible ink cartridges, photocake refill kits and our expertise you can deliver the perfect photocake (photo-on-cake) in just a few minutes.

Its not a RICE Paper photocake or POTATO Paper photocake(Also called wafer paper). Our edible photocake Icing sheets also called Frosting Sheets are made from Corn starch, sugar and don't add that EXTRA THIN layer to the cake, they just melt into the cake topping with in minutes.

Made from the finest quality ingredients available our edible photocake Icing sheets / Frosting sheets are of "FOOD GRADE". You have nothing to worry about. We are certified by APEDA, India for export.
With this new and latest “Photocake DIGITAL SERVICE” you can have a dream photograph, sketch , greetings or a image on cake starting at just Rs. 34*/- (US $ 0.8) onwards. ( estimate for a A4 sized medium quality printout). Now with this simple photocake technology you can now personalise Cakes , Cup cakes, Pastries and lot more.....Our photocake Icing sheets can be used on Butter, Fresh and Rich cream cake toppings.
We are the only manufacturer of photo cake Icing sheets in India meeting all global standards. Call us now.....we are now offering fantastic rates for this festive season. Mail us for more information.

OUR PRODUCT RANGE: "PHOTOCAKE: "ICING SHEET" for Photo on cake, "CHOCO SHEET " and "TRANSFER SHEET" for chocolates.

"Choco sheet" : Inkjet photo transfer on to white chocolates. Send a mail for more information
"Transfer sheet" : Regular and Custom pre-printed transfer sheets for Milk, Dark / Brown and White chocolates.
With just US$ 15.00 as setup fee we now print transfer sheets for even small orders of 50 no's. Now print as you like on transfer sheets. Your messages, Logo's and greetings absolutely anything you wish on Dark / Brown Milk and White chocolates. Customize chocolates like never before.
Send us a mail for more information
Test reports : Icing sheets US Format , UK Format and AU format.
Source(s):
http://www.wisegeek.com/what-is-a-photo-cake.htm
http://www.icingsheet.com/

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There is a Kwikscan scanner/printer that you can buy. However, you will have to purchase special edible inks that are printed on a rice paper made for this exact process. Once it has been printed you cut it to the form you want and gently place it on the frosted cake. The rice paper dissolves and the ink "melts" into the frosting leaving you with a perfect picture
Source(s):
http://www.kopykake.com/pc_kwik_kopy_systems.html
http://answers.yahoo.com/question/index?qid=20080119225445AADGilU

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