He looks at the picture.
Here what is 'at the picture'/'picture'? Is it Adverb, since it asnwers the question 'where'.
A NON-TEACHER's ANSWER
Hello, Rambharosey:
(1) I believe that you are correct.
(2) "at the picture" is being used as an adverb.
(3) But books call "at the picture" a prepositional phrase because
it has a preposition ("at") and it has one or more other words ("the
picture").
(4) And, as you suggested, "at the picture" is used to modify the
verb "looks." Thus, we can say that the prepositional phrase
"at the picture" is being used as an adverbial modifier of the verb.
(5) If I say, "He looks carefully," then it is correct
to say that "carefully" is a true adverb. I think that your teacher
would be happier if you referred to "at the picture" as a prepositional
phrase. I think that your teacher would also accept "adverbial
phrase." But it might be better NOT to call "at the picture" an
"adverb." Many books suggest that we use the word "adverb" for
single words (such as "carefully"). Oh, by the way, if I say, "He looks
with great care," then we are back to a prepositional/adverbial
phrase, not a true "adverb."
Sincerely yours,
James
Despite superficial structural similarity to a prepositional adverbial such as 'at the hotel' in
[1] He is waiting at the hotel.
most grammarians, nowadays at least, would probably not reckon 'at the picture' in your sentence an adverbial, essentially on the grounds that, whereas the (true) adverbial of [1] specifies where he is waiting (a quintessentially adverbial concept), 'at the picture' clearly does not really specify where he is looking so much as what he is looking at.
For this reason 'look at' would be labelled by most contemporary grammarians a phrasal verb, and 'the picture' simply as its object.