[TYPO3] png true color for click-enlarge-pic

[Philipp Albig]

Hi!

I set
[GFX][gdlib_png] = 1
[GFX][png_truecolor] = 1
so every image is rendered as png. But not in truecolor. For normal 
image this isnt a problem, because I  set
tt_content.image.20.image_compression = 39
in setup, so that every image is rendered as 'PNG', which is truecolor 
and is working fine.
But where can I define (or where is defined), in which format the 
'click-enlarge' pic is rendered?

Greets and thanks for any hints, Ph.

typo3 4.1.1