Normaly on the PS3, without any sort of external device, you could press the PS Button + Start Button at the same time for a screenshot, which would then be saved to Photos > Screenshots. Not all games allow you to take screenshots, however. It's really at the discretion of the developer. They can choose to opt out of allowing screenshots at all.
On the external side, you have a couple of options. You could connect your PS3 to your PC via USB, and use media player to take and save screenshots, which youcould then trasfer back to the PS3 ( This will require HDMI ). There's also a program called Gigapocket, although according to the PS forums, this method may be outdated now. There may be other ways, but im not too sure. I dont really take screens on PS3 as it is lol :P
Edit: I tried posting a URL for that Gigapocket program, but it says URL removed. How doya post link on here anyway? ^^;;
Edited, Jun 21st 2013 2:38am by Swiftskye
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