Before you toast the bread (and you will have to use a toaster oven or the broiler, not a pop-up toaster), use some tin foil out of which you have cut the shape you want to be toasted.
You can get the foil to stick to the bread by coating it with a little butter or mayo. If your customized design requires more than one toasting, make sure the edges of the toast are well-covered by foil to prevent burning (or cut them off before serving)
As if these fancy toasts weren't enough, how about this selfie toaster? Get it here..