You will need to install a user script manager extension to install this script.
This userscript is hosted at https://greasyfork.org/scripts/631-image-viewer
Which is synced with the gist over at github.
This means that it'll always be at the bleeding edge.
To use this script, inject it into any page, and RIGHT-CLICK on the image you want to view full-size
This can be accomplished by
while the image is "open", you can drag it around with the mouse to pan instead of scrolling
In Firefox, it's a little different. You have to click the image to toggle the dragging functionality on and off
There is a close button, and a full URL link at the top of the page
You can press Esc to close the image without the need for clicking the close button
You can press Ctrl to re-initialize the script, for when dynamically generated images don't respond to right-clicking
If you left click an image spawned by the script, and press the arrow keys, you can resize the image. Left/right for width, and UP/Down for height
Special thanks to Cyberdevil for providing suggestions