This would be a device you set a cake in, and it would spray whatever picture you put into it onto the top of the cake.
It would work like an ink jet printer, except the printing cartridge and nozzle would move.
The physical device and mechanism would be pretty trivial to build.
The only difficulty is creating an easy to clean cartridge system and nozzle.
The nozzle would have to be detachable, for easy cleaning.
You could either use a colored sugar water as the base colors.
You could either plug it into a computer or sell cartridges.
IF you are going for the razor blade scenario, obviously would would want cartridges.
I would like to make money form the device, and some supporting official images, but still let them plug the device into their computers USB2 port.