If you are creating your own presentation, there are endless ways you can find photos to use, especially on sites that offer cheap royalty free images. But, you must be certain that the resolution is clear enough once you obtain or download that photo. Basically, photos are made up of tiny pixels. A 'dots per inch' value is then given to the photo, which tells the viewer more about the resolution. More recently, an industry standard has been created; it is 300 dots per inch, or dpi. If you choose a photo that is at least 300 dpi, you can usually be assured that the photo resolution will be clear enough for your use.