A design image is combined with a product image to create a composite image to illustrate the result of printing the design image onto the product. To create the composite image, the opacity of each pixel in the design image is set according to the brightness value of the pixel such that completely dark pixels are opaque, completely white pixels are transparent, and pixels having intermediate brightness values are assigned intermediate opacity values based on the brightness of the pixel. The design image pixels and the corresponding product image pixel are blended according to the opacity values of the design image pixels to create the composite image.