On a PC
Hold the Ctrl key and push the "Print Screen" or "PRT SC" button in the upper right-hand corner of your keyboard.
Then, open an email or click reply to an existing email and use CTRL+V to paste the image into the body of your email. If that does not work, you can open a program that handles pictures, like Paint or Word, and paste the image into the document.
On a Mac
Hold Shift + Command + 4 and you will see "crosshairs" - drag these around the item you want to take a screenshot of and this will be saved automatically on your desktop. Attach this to your email.