This is an old school technique called buttercream transfer.
First you will need to take a picture from a coloring book or a line drawing and tape a piece of wax paper over the picture, tape both, to a piece of cardboard so that it will be easier to work with all the movement. It is important to note: make sure your picture is in reverse so the finished picture is correct.
Next, use a No. 2 round tip, pipe the entire outline and fill in anything that should be black. Place the outline in the freezer for an hour, this will give you a much easier time of filling in your colored buttercream. Starting with the lightest color fill in the outline with a larger round tip (you may need to return the picture to the freezer from time to time to allow the colors to set up before adding another color.)
This is the finished fill in of minnie. You can freeze this until needed.
When ready, frost your cake before removing your buttercream transfer from the freezer. Gentle place your picture in place on the cake and using your finger gently press over the picture. The heat from your hand will soften the cold buttercream. Leave the picture set a few minutes so that it warms up (about 5 minutes), then gentle peel off the wax paper.
Your picture frosting will be attached to the cake and you should have a clean wax paper.
Finish your cake with boarder pipping, wording and whatever touches you would like.