Hints: some of the hints are not specific,but they give you a general idea as to which episode the screencap is from..
1: doubt you need one for this, but just in case- it's from the last few minutes of "Chuck Versus The Other Guy", where Chuck tells Sarah he loves her x4
2: Chuck did NOT kill Julius
3: "You want something done right- you do it yourself!"
4: Pesticides can lead to low sperm count?!
5: Chuck breaks up with Sarah, not knowing she lied to him about how she feels
6: Chuck you sneaky, sneaky guy!
7: "I need pants!"
8: Morgan looking go-oood! since when is it so windy at the Buy More?!
9: Nobody tells Sarah what she can, or cannot do- got that 49- B?!
10: "the Carnivore" is chasing Chuck and Jill as Sarah and Casey come to the rescue
11: Chuck leaves his Stanford dorms
12: Chuck working on his spy- will
13: oh Emmett, couldn't you just stay in the car?!
14: Sarah tells Chuck she loves him (FINALLY!!!)
15: Africa?!?!?!?!
16: Morgan Grimes- the great romancer
17: Chuck doing what he's good at, a la season 1
18: What happened in Prague?!
19: Getting Ellie's wedding present- part 1
20: Driving Miss Walker- DC style
Answers: 1-C, 2- B, 3- A, 4- D, 5- A, 6- B, 7- B, 8- C, 9- A, 10- A, 11- D, 12- D, 13- A, 14- C, 15- D, 16- C, 17- B, 18- A, 19- D, 20- A