impression vs mental image

impression and mental image both are nouns.

Word	Noun	Adjective	Verb	Adverb
impression	Yes	No	No	No
mental image	Yes	No	No	No

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

Other hypernyms of impression include image.

impression (noun)	mental image (noun)
an impressionistic portrayal of a person	an iconic mental representation
the act of pressing one thing on or into the surface of another
(dentistry) an imprint of the teeth and gums in wax or plaster
an outward appearance
a vague idea in which some confidence is placed
a clear and telling mental image
a symbol that is the result of printing or engraving
all the copies of a work printed at one time
a concavity in a surface produced by pressing