collage vs photomontage

collage and photomontage both are nouns.

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

As nouns, photomontage is a hyponym of collage; that is, photomontage is a word with a more specific, narrower meaning than collage:

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

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