Here is a guide to the screencaps that have been added to this spot so far:

Season 1 Episodes:
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Season 2 Episodes:
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2x23 ~ 'Over There' (Part 2)


Season 3 Episodes:
3x01 'Olivia'
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Season 4 Episodes:
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NOTE:
- People have not keyworded their screencaps, so I can't link you to all of them. The ones that have keywords I have linked to, but they do not seem complete.
- I intend adding the remaining season 2 episodes. If anyone intends adding any screencaps, please let me know.