Printing a photo onto a cup is not difficult, but it does require attention to detail and multiple steps. Obviously, you are not going to send your cup through the printer, so you need to find another way to transfer the image. The secret is water-slide decal paper. Essentially, you print an image onto the decal paper and then use the paper to transfer the image onto the cup.
Match your decal paper to the cup you plan to decorate. Printers do not produce white ink, thus decal paper is available in both clear and white. If your mug is white, use the clear decal paper and allow the background of the mug to show through. If your mug is any other color, use white decal paper to fill in the white areas of the image.
Load your image onto a computer and prepare to print. Insert the decal paper into the printer. Consult the decal packaging to make sure the printer is formatted for the paper size. When selecting print, upgrade the quality of the photo to the highest print resolution.
Spray your image with an aerosol acrylic spray, available at most craft or home goods stores. Spray a single coat and allow the acrylic to dry. Then spray a second and third coat, allowing the acrylic to dry for 30 to 60 seconds between coats.
Cut the image from your decal paper and place it in a bowl of water for 40 to 50 seconds. When the paper is removed from the water, the image should slip off the decal paper easily. Rub a small amount of decal mounting fluid on the area where you plan to apply the decal to the cup. This will provide a stronger adhesive surface. Apply the decal to the cup immediately and remove the backing. Gently apply a second coat of decal mounting fluid over the top of the decal.
Press a squeegee or sponge over the image gently to remove air bubbles or creases on the decal. Once the image is smoothly placed on the cup, allow the decal to dry for at least three hours. Once dry, the image is safe to wash.