(1) "I had a picture in my mind of him to drive driving his car up a wall".
(2) "In my mind, I expected him driving to drive his car up a wall".
When a clause is object of a preposition like "of", as in (1), it is normal to use an ing- form of the form. This is sometimes called a 'gerundial clause', but 'non-finite clause' is fine too. Using the ing-form implies some action in progress (like the progressive "he was driving his car up a wall"). But with verbs like "expected", you'd expect to see an infinitive verb-form as in (2).
(3)"The reader can see what the paragraph is going to be about and is, therefore, better prepared to understand it".
In your other example (3), the subject of "is going to be" is the noun "the paragraph". The underlined element is a noun phrase containing a relative clause. "What" means "the thing which", and "the thing" is clearly a noun, so it means "the thing which is going to be about".