Lien direct : http://www.picnik.com
http://mozillalabs.com/jetpack/2010/05/01/using-jetpack-sdk-0-3-context-menu-api/
Add-on Firefox : https://addons.mozilla.org/en-US/firefox/addon/picnik/

Outdated : https://addons.mozilla.org/en-US/firefox/addon/158233/