How do I screencap??

I've found a website, with lots of lovely photos that will come in handy for *cough* reference shots *cough*, but when I 'right click' on something to 'save' it, a little window pops up and says that it's property of ...blablabla.

I *know* there's a way to save a copy of the entire screen, but I dunno how.

*pleads*