Cake decorating has progressed quite a bit from the days of piped-on "Happy Birthdays" and icing rosettes. You now have tools available that make personalizing cakes with your own images or transferring them onto your cake a breeze. For detailed, lifelike images, you can have edible icing sheets printed that you simply affix to fondant or buttercream, and for simple designs, such as cartoon characters, you can employ a "fill-in" method using buttercream. If you don't have much time or you have an intricate design, use transfer sheets; if you have a few hours to spare, try filling your design in yourself.
Print out your selected picture on a regular printer the size you want it to appear on the cake. Trace the image on a sheet of parchment paper using a fine tip marker. Mark the outer boundary of the cake as well.
Turn the parchment paper over on a rigid work surface you can freeze, such as transparent plastic or acrylic. Secure the parchment to the work surface with tape.
Insert a No. 1 circle pastry tip in a pastry bag and fill the bag with black buttercream icing. Close the top of the bag with a twist tie.
Draw the outline with the black buttercream. Place the work surface in the freezer every few minutes to keep the buttercream firm while you trace to prevent smearing.
Fill one pastry bag for each color you need with colored buttercream. Use a wide L-19 pastry tip for the colors you'll apply to large areas for ease of application.
Fill in the colors on the parchment paper; continue placing the tracing in the freezer every few minutes, again to keep the buttercream from smearing. Apply background colors last.
Hold the work surface up to the light to identify weak spots. Use a fine paint or pastry brush to nudge the buttercream to the edges of the outline as needed. Fill in any bare spots as needed.
Apply the background color for the picture; use a color that blends with the color of the cake. Smooth the background with a narrow offset pastry spatula and freeze the buttercream picture overnight.
Frost the cake when you're ready to apply the transfer. Take the transfer out of the freezer and remove it from the work surface.
Position the frozen-buttercream side of the parchment paper on the cake and gently press it into the icing. Peel the parchment paper from the frozen buttercream.
Moisten a thick lint-free paper towel with cold water after the buttercream thaws on the cake, about 20 minutes. Gently wipe the lines from the piping process from the buttercream to create a smooth surface on the transfer.
Edible Icing Sheets
Peel the plastic backing from the edible icing sheet and cut the design you want to apply to the cake from it using scissors. To cut straight edges uniformly, use a straight edge and razor blade or a guillotine paper cutter.
Apply a thin layer of gum-paste glue to the fondant using a pastry brush; if you have a buttercream icing, you don't need gum-paste glue.
Position the transfer on the cake with the side you removed the plastic backing from facing down. Lightly press the transfer to affix it.