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.
"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.
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.