You will need to install a user script manager extension to install this script.
Move your mouse over a pin, and press the 'z' key to open the largest (original) image Pinterest has for that pin. By default it will open in a new tab in the foreground. Pressing 'Shift-z' will open in a background tab.
You might be surprised how big some images are compared to the "close up" image that you get when normally clicking on a pin :) This is especially true when an image is more horizontal (landscape) than vertical.
Requirements: Tampermonkey or Violentmonkey on a recent version of Firefox or Chrome. Greasemonkey probably won't work properly with this script (unless they fixed a problem with Object.keys()). Safari is not supported since I'm on Linux, and can't easily test it.