Download the caps instead of saving them one at a time from the Gallery:
Note:These caps are 1280x720.

Season 1 (Castle and Beckett only):
1x01 - Flowers For Your Grave
1x02 - Nanny McDead
1x03 - Hedge Fund Homeboys
1x04 - Hell Hath No Fury
1x05 - A Chill Runs Through Her Veins
1x06 - Always Buy Retail
1x07 - Home Is Where the Heart Stops
1x08 - Ghosts
1x09 - Little Girl Lost
1x10 - A Death in the Family



Season 2:
2x01 - Deep in Death
2x02 - The Double Down
2x03 - Inventing the Girl
2x04 - Fool Me Once
2x05 - When the Bough Breaks
2x06 - Vampire Weekend
2x07 - Famous Last Words
2x08 - Kill the Messenger
2x09 - Love Me Dead
2x10 - One Man's Treasure
2x11 - The Fifth Bullet
2x12 - A Rose for Everafter
2x13 - Sucker Punch
2x14 - The Third Man
2x15 - Suicide Squeeze
2x16 - The Mistress Always Spanks Twice
2x17 - Tick, Tick, Tick...
2x18 - Boom!
2x19 - Wrapped Up In Death
2x20 - The Late Shaft
2x21 - Den of Thieves
2x22 - Food to Die For



Season 3:
3x01 - A Deadly Affair
3x02 - He's Dead, She's Dead
3x03 - Under the Gun
3x04 - Punked
3x05 - Anatomy of a Murder
3x06 - 3XK
3x08 - Murder Most Fowl
3x09 - Close Encounters of the Murderous Kind
3x10 - Last Call
3x11 - Nikki Heat
3x12 - Poof! You're Dead
3x13 - Knockdown
3x15 - Lucky Stiff
3x15 - The Final Nail
3x16 - Setup
3x17 - Countdown
3x18 - One Life to Lose
3x19 - Law and Murder
3x20 - Slice of Death
3x21 - The Dead Pool