Detection of fake images by forensic image experts can reveal whether a photo has been edited and can often show the areas within a photo that the manipulation occured. With the advances in technology from software such as Adobe Photoshop, the ease of manipulating or "photo-shopping" photos means edited photos have become the norm and not the exception. It can be used for harmless fun, but also for more malicious reasons, and it is the later we enjoy exposing.
We can detect fakes photos and if original photos have been manipulated through techniques such as Double-JPEG compression, changes to the marker sequence and differences in the hex structure. There are so many techniques at our disposal that the odds of getting an edited photo past us are incredibly low. It can also be important to authenticate a photo to prove that no manipulation has taken place if questioned by other parties.