Balteus

Implement a brightness setting value, when an image is dark (do nothing) or a image is bright (invert), like this code: http://jsfiddle.net/s7Wx2/7/ and invert the image, like this code: https://github.com/vycb/InvertImage/blob/master/main.js

Even photos should be inverted (it is a user decision for that website). Maybe you can add a shortcut or right mouse event to set some selected (or only current)...

An easy way is also to implement a mouseclick (STRG / SHIFT + left/right click on a picture...) event to invert/normalize one picture, Usually a big picture above or under...