The golden rule for printing is that your images should have a resolution of about 300 dpi (dots per inch). So for the dimensions of your 13 x 18 Photo Prints, you should aim for images of (7 x 300) x (5 x 300) = 2100 x 1500 dpi for optimum quality. Images that are just 3.15 megapixels means your phone or digital camera is perfect! Exact dimensions can be found here.