This photograph of Wilhelm Conrad Roentgen (discovered x-rays in 1895) is a good example of an image; consideration of this photographs characteristics can illustrate general points pertaining to medical (and other) types of image. At any point, the image has an intensity ranging from "white" to "black". The intensity value reflects the amount of light that was incident onto the photographic film used to acquire this image. The greater the amount of light that is incident on any given location, the whiter (brighter) the appearance of this location in the resultant printed image (and vice versa).
The line AB through the image (above) has varying values as depicted by the profile AB (right). An image is generally a depiction of a two-dimensional variation of a selected parameter. In the case of a photograph, it is the light intensity incident on the photographic film that varies from location to location.