Noise problem with test online

Dear All,
 
Ok I still don't understand the "noise problem" I have with GD resize.
You can find my test online at this url : http://www.kaipifraise.com/GDTest/
The problem is very simple : I take a white picture, I resize it, and I don't get a really white picture. You can see it if you have a calibrated screen or if you change the brightness in Photoshop or if you just take a look to the size of the picture which increase !
The problem seems to appear only for some ratio of resizing but I can't understand...if anybody understand that I would appreciate to have an explanation..
 
Best regards.

 
_________________________________________________________________
Inédit ! Des Emoticônes Déjantées! Installez les dans votre Messenger ! 
http://www.ilovemessenger.fr/Emoticones/EmoticonesDejantees.aspx