Maybe you can do an edible image of it and then paint over it with lightly colored piping gel? Paint blue gel over the blue parts, yellow gel over the yellow parts, etc. That way you could see thru the gel and it would pop and not be as boring as a regular edible image.
By the way, I have never done this but it sounds like a good idea.
First you make sure the image is the correct size for the cake you are putting on. Then you place your rice paper sheet on top of the drawing and trace it with black edible marker, Then you fill it in with edible colored marker, just like you are coloring in a coloring book.
I have never tried painting with gel.
Well I hope it works for you!
I frost all my cakes with a non dairy whipped topping Called Pastry pride. It comes frozen and all you have to do is thaw it and whip it up. It is sold in California at Smart and Final, but you can also by something similar called Bettercreme and also Rich's. Look for them so you won't have to stabilize your own.
Here is an article on 6 different ways to stabilize whipped cream, maybe it will help
What about making some buttercream roses? Do you have ingredients to make BC?
Annabakescakes has given you some great ideas. I would also try to mop up some of the milk that has leaked with paper towels. Then add a simple shell border to the bottom to cover up and fill that empty space. I agree on trying to re write the message and make it smaller. Do you know how to make simple flowers? You could add some to the corners.
Try to use less milk next time so it won't leak as much.
I would use a tip 2 or 3 for the outline. For the color flow, you can use a bigger tip, or just snip a tiny bit of the end of your bag.
Make sure your outline has crusted before filling in, or it will all just run together. I let mine sit about 30 minutes or so.