When I want to add an image to the top of a cake I like to make reverse chocolate transfers because they are pretty easy and generally turn out pretty well. They are perfect for the top of a sheet cake; they're low maintenance in terms of delivery and can be whatever size you need.
Here's how to make them:
Start with the image, adjusting the size and orientation to fit the cake. If there is writing as a part of the image, reverse it before printing it out. I've also just drawn out what I wanted to use for the image, like the cards for the top of this cake and the number 30 for the top of this one.
Place the image on something fairly sturdy that will support the chocolate piece so you can move it when you're done. A clipboard or a folder work well. Tape a piece of waxed paper over the image (parchment paper also works but is harder to see through).
Prep the chocolate one color at a time. It will cool faster than you
can get to the next color, so it just wastes time to get all the
chocolate ready before starting. This is not the time to use your fancy, expensive chocolate because it doesn't melt as well as the candy melts or the chocolate bark.
I use plastic tips when I pipe chocolate because the temperature matters and I often have to put the piping bag into the microwave for a few seconds to reheat it. I just fold the whole piping bag over and put it into a jar or a glass so it won't drip all over the place and heat it for a few seconds at a time.
It helps to have a test area on a plate or paper towel to make sure the chocolate is coming out at a manageable rate before you start working on the image. If it's too slow, heat a bit more. If it's too fast (read: unmanageable), let it cool a little.
Use the chocolate to trace the outline of the image and let it set up for several minutes. The first pass should be all of the detail work. It's a little bit hard to adjust to, but you have to work backwards because whatever goes down on the waxed paper first is what will be on the top of the image when it's flipped over.
Fill in the background color(s) over the outline (this is just like flooding with royal icing). Take your time; air bubbles are always an issue with this technique and are exacerbated if the flooding is done too quickly. When it's all filled in, you can use a toothpick to smooth things out. I don't do too much of this because the warmer chocolate can pull through the already cooled outline colors and drag them. I don't care if the back is pretty bumpy and it generally isn't worth the risk.
Refrigerate the entire chocolate piece for about 20 minutes. (Don't skip this step. Without it, the chocolate will be more fragile and won't stand up well to any handling to place on the cake.)
Then just turn it over to place on your cake when ready. It will be a little bit uneven, so I just add some frosting under it where necessary.
These can be made several days ahead of time and stored in a cool, dry place - I usually layer them between parchment paper in a tupperware container and leave it in the fridge.