What is the subject of these sentences?
How much do you remember from that display?
You or display
Near the satellite model was a large picture of Jupiter.
Are you searching for the gift shop?
1. Display is the object of a preposition -- so it is not the subject. You is the subject.
2. What was?
3. Who are searching?
So picture for the second one
And you for the last one?
Yes; both are right.
To determine the subject of a sentence, you need to identify who or what is performing the action or being described.
In the sentence "How much do you remember from that display?" the subject is "you" because it is the person being asked about their memory.
In the sentence "Near the satellite model was a large picture of Jupiter," the subject is "a large picture of Jupiter" because it is what is being described as being near the satellite model.
In the sentence "Are you searching for the gift shop?" the subject is "you" because it is the person being asked about their activity of searching.