Like other visual aids, images or photographs should be clear, bright, and of adequate size. If possible, crop large images to include only the relevant parts. Cropping reduces extraneous information, and you may be able to enlarge the cropped image for easier viewing.
It's also important to remember that just because a person can see an image doesn't meant that they understand what's going on in the image so keep that in mind when it some to visuals. Given students time to really understand what's in the photo.