Well it depends on what your trying to do. Are you just trying to create an image on the cake? There are several methods, you could print out the image and using a needle tool or dressmakers pin, make small pin pricks to outline the image thru the paper and then go over it, or you could do a buttercream or chocolate transfer and then place it on the cake. You could also use a projector to project the image onto the cake and the trace it.
HTH
Jenn
Before I got my projector, I transfered pictures using plastic wrap. I would lay a sheet of plastic wrap over the picture and outline it in marker. The cake would be in the freezer. Once the icing was frozen firm, I'd lay the plastic wrap on the cake and trace the lines using a toothpick, which would imprint the design into the frosting. This didn't work great with intricate designs, but for most things I did, it worked fine. If the design was a large one, I would have to put the cake back in the freezer to keep the icing frozen. And I always chilled it again before lifting off the plastic wrap.
When I took the wilton classes, my instructor taught us to use piping gel. You take a piece of wax paper and lay it over your picture, outline the picture with the piping gel, after your cake has crusted over turn the wax paper over (gel side down) and lay it on the cake carefully so that it does not move. Lightly rub it with your finger, then gently pull it away from the cake. the gel will leave the design on the cake and you can just decorate over it. (Remember the picture will be facing the opposite way on the cake so if you want it the way it is looking in the picture then you will have to outline the opposite side of the paper the picture is on then transfer to the wax paper...make any sense?)I use to use the ole toothpick trick before she showed us this and oh how easy it is. 
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