Yes, I tried that. But it's kinda strange. It looks like TG is trying to work on this reduced size picture only and after zooming 3600 x 2400 picture by, let's say 200% - it's pixelated.
EDIT:
"When rendering is completed the image is always displayed at full size. If it was larger than could fit on the screen it will initially be zoomed to fit on the screen. You can then use the zoom tools (see the View Manipulation section below) to display it at full size if so desired."
Yeah - now I get the idea ;-)