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 test it.

Known issues:

• "Visually similar results" page won't let you view the original images. There is a work-around I could do (basically replacing "736x" with "originals"), but it's not guaranteed to work, so I'm hesitant to add it.
• If you're not hovering over an image and press 'z', it will open the very first image in the pin grid.
• There is a possibility that instead of opening the image in a new tab, it will ask if you want to download the image. For quite a while now it hasn't happened to me, so maybe it was a browser or Tampermonkey bug. Make sure everything is up to date if this happens to you.

Notes:

• Pressing 'z' on a close-up image also works.
• I've only tested this with the Tampermonkey add-on on recent versions of Firefox and Chrome. If you're using something else and it doesn't work, let me know, otherwise I probably won't fix it.