Screencap Master List
Here's a list of sources where you can find screencaps of your favourite show, if you should ever need them to use for your challenge icons.
If you know any other places that aren't listed feel free to leave a comment below with a link.
Screencap Master List
24 TV - a site providing general screencaps of the show
24 Fans - a site providing general screencaps of the show
Daydreaming - includes various photoshoots/promos, as well as screencaps of select episodes.
Kiefer Sutherland Fan - dedicated to Kiefer Sutherland incl. caps of Jack Bauerfrom the show
Almeida Is God - Screencaps centered around Tony Alemeida/Carlos Bernard
Tony Almeida - He's Not Dead - dedicated to Tony Almeida/Carlos Bernard, but including pictures of other cast members
helga_1987 - a fully comprehensive set of Day 7 Screencaps here
Katee Fan - screencaps of Katee Sackhoff in her role as Dana Walsh