Oh yea.... copying and pasting probably won't work if you're going from an external document or picture. BUT there is a way to make it work!
You can either crop them in Preview, OR just take screenshot (command+shift+4) and insert it from the desktop. An alternative would be to hold down the OPTION key while you take the screenshot (which will copy it to the clipboard) then all you have to do is paste it into your document! Here are those options in action: