I would opine that many native speakers would say
In this picture I can see two people.
There is no need to start with a list, The first thing... especially if the photo is detailed, but the OP wants to know if using the plural verb in this instance is grammatical or not
The first thing [that] I can see {in this picture} are 2 people
If we take away the th-words (that, this, there) we are left with
WRONG: The first thing I can see are two people
Although it doesn't sound wrong because the verb agrees with the predicate plural noun, two people; it disagrees with the subject of the sentence which is "The first thing". In fact, it follows the standard Subject Verb Object (SVO) order
CORRECT: The first thing I can see is two people
The plural "two people" after the singular verb makes it sound a bit strange even though "two people" is not the subject of the verb. But verbs MUST agree with their subject.
- • “The first thing” is a noun phrase which is the subject
• “[that] I can see” is a reduced relative clause
(Since the relative clause has a subject (I) already, the relative word, that, is unnecessary and may be omitted.)
- • “in this picture” is a prepositional phrase, and it is the complement of the verb "see". The verb “be” is the copula or linking verb.