After few search I didn’t find any good way to take a screenshot of an application, except loading this application in a browser and take that damn screenshot.
Searching a while, I found python-webkit2png. Actually one of the fastest one, using Qt-webkit as backend.
With this, I could achieve nice screenshot and a good overall image quality, but my project was not a python one, and the system does not broadcast interface for external system to plugin.
So I decide to make a simple python server to handle this utility, and expose this to the world : python screenshot server.
Feel free to use it, I personnaly run it behind another web server, with success !