I would use either a fondant plaque or a frozen or chocolate transfer.
the fondant plaque will give you more time to create the design and a very stable medium.
I say you take a picture and print it out as large as you need it. If you have a program that will take and image and print it out (overlapping) on several sheets. Like "printArtist" or sometimes you can do it in paint.
Then I would softly trace it on top of your fondant and then paint it up, unless you want to marble it before hand.
What is a fondant plaque? How do you make one?
This is due for Sunday. Is there a tutorial for Fondant plaques somewhere? I can't find one.
Just cut the shape out of fondant, trace the design on or whatever method you are choosing to do, and lay it on top of the cake.
Here's what I would do .. print it out to whatever size you need (make sure you "mirror image it" ) Then trace it onto wax paper with a black marker, Ice your cake and make sure it crusts, after that trace the wax paper image with piping gel, flip it onto your cake and press gently. You will then have the image on your cake and you can trace over the piping gel and color it in with your icing.