Uh, a thermal printer (whether it uses heat-sensitized paper, like a traditional fax machine, or a thermal cash register printer, or a thermal theatrical ticket printer, or whether it uses a thermal ribbon, like the old ALPS MicroDry system) is a completely different animal from a thermal inkjet head, which uses heat to force the ink out (apparently by boiling off some of the solvent).
At any rate, I'm perfectly content to continue outsourcing edible printing, optical media printing, and any other printing that has to use inkjet technology, because I refuse to own an inkjet printer, or permit one in my home.
When I did the Leland Awards cake, I used an ordinary (but very clean, the same pair we keep in the silverware drawer, for opening food packages) scissors to trim the picture of the trophy to the very edges of the trophy itself, removing all background, and the International Printing Museum logo to its outline. I did the same with the speed limit sign on my parents' 55th anniversary cake.
Since it wouldn't be very practical to do the same with most of the images I have planned for my own 55th birthday cake (public domain photographs of Squad 51 and Engine 51 from the old Emergency! TV series, and my own photograph of Los Angeles County Fire Station 127, which was used for all the Station 51 exteriors), I plan on just trimming them to the very edges of the photographs, and maybe piping a border around them.
As to an ink tank with a sponge holding more than one without a sponge, that flies in the face of the laws of physics: matter takes up space. And two objects cannot occupy the same space at the same time. The only exceptions I know of are cases in which molecules fit together (e.g., if you combine a liter of 200 proof ethanol with a liter of pure water, you get slightly less than two liters of 100 proof ethanol), or where a gas dissolved in a liquid takes up less space than the same amount of gas can be safely compressed into by itself (e.g., acetylene tanks are filled with a porous wadding that is soaked in either acetone or dimethylformamide, because acetylene explodes spontaneously under high pressure)