stock image vs photo

stock image and photo both are nouns.

Word	Noun	Adjective	Verb	Adverb
stock image	Yes	No	No	No
photo	Yes	No	No	No

As nouns, photo is a hypernym of stock image; that is, photo is a word with a broader meaning than stock image:

stock image: an existing photograph licensed for a specific use
photo: a representation of a person or scene in the form of a print or transparent slide or in digital format

Other hypernyms of stock image include exposure, photograph, pic, picture.

stock image (noun)	photo (noun)
an existing photograph licensed for a specific use	a representation of a person or scene in the form of a print or transparent slide or in digital format