If you see a picture you like online, right click it and select copy image (Firefox), or hold the mouse down on the picture until a menu appears and select copy image (Mac Internet Explorer).

If you want to copy simply what you're looking at on the computer, whether it be offline or online, press "Print Screen" on your keyboard or if you want to copy a certain program's image off separately, press "Alt-Print Screen" together. If you want to copy text, the short key is "Ctrl+C" pressed together, that is, after you've highlighted the text you want to copy.

Hope this helped!