Option 1: Right click the image > open in new tab > replace "ku-xlarge.jpg" (or whatever the last part is) with "original.jpg" in the url > hit enter
Try it on the image above to see what is happening in the image.
Here is a wall paper sized image to try it on:
Update #1: Another option for images in the middle of a post (not header or comment images): click expand in the bottom right corner > open in new tab. This should bring you right to the full sized image. Thanks to Rosetto Viacava for reminding me of this one.
Update #2: Both Options 1 & 2 above work with images on the front page as well.