Thank you,
You were right i looked more deeply in the way that the inventory is generated on the GUI and found this method :
renderItemIntoGUI(ItemStack stack, int x, int y)
in the ItemRenderer class and it is really easy to use. For example i use it like this :
public static finale Minecraft MC = Minecraft.getInstance();
MC.getItemRenderer().renderItemIntoGUI(MC.player.getHeldItemMainhand(), 200, 200);
For some reasons when i first read the ItemRenderer class i completely passed over it.