There are a few ways to do that. You can cover your metal with white gouache (watercolour) and transfer your picture using a piece of carbon paper. Clean the surface of your metal to make sure that the paint spreads evenly on it (wash it with pumice powder or heat it up slightly to burn the grease off).
Another way is to put a picture on your metal and emboss enough dots along the lines of your drawing that you would be able to connect them with a scribe afterwards.