Shows the higher res version of an image, if it exists.
Tumblr loads images in 2 resolutions: 500 wide and 1280 wide.
Some tumblrs actually have the 1280 resolution, but when clicked, they link to the 500 one.
This extension attempts to automatically find and display the bigger image, if a higher resolution is available.
UPDATE: Now works with gifs! [from 250/400 to 500 resolution]