[Typo3] Graphicsmacick: low quality of PNG thumb

[Francesco di Francia]

Hi,

i use templavoila and graphicsmacick.
In TV DS i set an IMG w+h field, so typo3 know that if editor choose a 
larger image it must resize it before rendering on the webpage.
All works ok but the quality of the PNG thumb is very low. Becauseof the 
original image is a jpg i tried a workaround by settings to 0 the 
parameter $TYPO3_CONF_VARS["GFX"][thumbnails_png], so that the thumb 
will be a png only if the source is not a jpg.
Anyway it still create a png thumb.
How i can obtain a better thumb ?
Looiking at the thumb i think it's a matter of low number of color, 
maybe it creare an 8 bit PNG.

Best regards.

Francesco