I think there is a good chance that a new word or phrase has been coined to mean fake photograph.
Is there a single word to express it?
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Normally if a photograph has been faked it is said to have been Photoshopped, or called a Photoshop, after the popular image editing software.
Sometimes this is shortened to shopped or shop, as in:
That photo's a shop [or been shopped], I can see the pixels!!1
More general terms include forgery, counterfeit and of course fake.
CG is a term that could be used for a created (or faked) photo. It is short for a computer-generated image. "CG" is not exactly equivalent to "fake," but CGs are often contrasted with "real photos."
This is an interesting site that lets you guess if a series of photos are real or CG: http://area.autodesk.com/fakeorfoto/challenge (I was so-so at the challenge.)
Instagram/Instagramming is another - "aura" enhancing - form of photo manipulation.
In old days, it was airbrushing. "Just airbrush the b------ out" a famous movie actress once told a fashion photographer who didn't know what to do with a photo that included her and her husband (a Pepsi executive) who had recently died. But even in the 19c clouds were "burned into" otherwise eggshell white skies. Red, blue or green filters were used to soften or roughen the appearance of skin.
"Digitally manipulated" as above, "post-processed" or "photomontaged".
It's strange, that no one has mentioned hoax yet. It's a bit broader term, because it is not used to describe photographs only.
The technical term is retouched.
From Oxford English Dictionary:
To improve or repair the appearance of (a painting, composition, photographic negative or print, etc.) by small alterations or fresh touches; to touch up.
1979 N. Farah Sweet Sour Milk v. 78 There was the photograph of the All-Powerful retouched to make him look much younger and handsomer.
In general usage (particularly within the print industry) the original meaning has been expanded to include photographic prints or negatives that have been significantly altered or changed from the original to constitute a fake photograph. A fake is clearly defined as having been manipulated to represent something that is not real.