It's getting easier and easier to create fake photos and sometimes it's pretty hard for the untrained eye to see whether an image has been "photoshopped" or not.
OE Magazine published a guide which helps you to discover fake photos.
Although generating fake images historically originated with darkroom tricks, today almost every fake image is made using a computer. Even though it is getting more difficult to discern a real image from a fake image as image processing software improves, image analysis can still be used to detect traits that can expose many of them as fakes. To understand how fake images can be detected we must first understand how they can be made on computers. The two most common methods today for generating fake images are to "paint" a new image outright or to alter an existing image that has been captured by a camera..