photomontage vs collage

photomontage and collage both are nouns.

Word	Noun	Adjective	Verb	Adverb
photomontage	Yes	No	No	No
collage	Yes	No	No	No

As nouns, collage is a hypernym of photomontage; that is, collage is a word with a broader meaning than photomontage:

photomontage: a montage that uses photographic images
collage: a paste-up made by sticking together pieces of paper or photographs to form an artistic image

Other hypernyms of photomontage include montage.

photomontage (noun)	collage (noun)
a montage that uses photographic images	a paste-up made by sticking together pieces of paper or photographs to form an artistic image
	any collection of diverse things