Print out the image on photo transfer paper. For most brands of photo transfer paper you will need a toner-based printer; not an inkjet printer. The image will be reversed when it is transferred to the metal, so use photo manipulation software to reverse the image before printing if this is an issue for you.
Clean off the sheet metal completely. Dry the metal with a clean rag.
Press the photo transfer paper to the sheet metal. Some photo transfer paper systems come with both paper and a chemical solution. If this is the case, squeeze the chemical solution onto the surface of the sheet metal before pressing the paper to it. Other decal style systems contain the solution on the paper already. Follow the instructions included with your photo transfer paper; some products must be completely dry before proceeding to Step 4, others must be placed in the oven immediately after the paper is applied to the metal.
Place the metal in the oven. Set the temperature for 400F. Follow the instructions included with your photo transfer paper. Some products do not need to be baked for long, others must be heated slowly up to 400F over the course of several hours.
Pull the metal out of the oven and place it in a bucket of water to soak. Once you see the paper begin to disintegrate, pull the metal out and rub off the excess paper with polishing paper. If your brand of photo transfer paper requires a sealer to be applied, apply this now.