If you want to make measurements on a photograph, the first thing you have to do is determine the scale of the photograph. If a picture is of a car, for example, obviously the car is not really just a few inches long as it appears in the photo. However, if you can measure that car in real life you can determine the scale of the photo and use that information to measure other items in the same picture.
Measure the real-life length or height of an object that is present in your photograph. For example, if there is a framed photograph in the background of the photo in question, measure the length or height of that framed photo as it exists in your home. Write this down in inches.
Establish scale by measuring the same object in the photograph itself. For example, if the framed photograph in the background of the photo in question measures 12 inches in height in real life and 1 inch in height in the photograph, your scale is 12 inches = 1 inch.
Use a tape measure to find measurements on your photograph. Use the scale you previously established to perform the conversion to actual measurements. For example, if a person is standing in the photograph that measures 6 inches on the photo itself, using the previously established scale you can determine that 6 inches multiplied by 12 inches equals 72 inches. In real life, that person is 72 inches tall.