Well if its just the defaults you want to change to png, well that's easy.
Edit templates\base\scripts\BxBaseFunctions.png
Change this at about line 260
return getTemplateIcon(isset($aGenders[$sSex]) ? $aGenders[$sSex] . $sType . '.gif' : 'visitor_' . $sType . '.gif');
To This
return getTemplateIcon(isset($aGenders[$sSex]) ? $aGenders[$sSex] . $sType . '.png' : 'visitor_' . $sType . '.png');
Now upload 64x64 medium male and female png images and 32x32 images to templates/base/images/icons. File names.
man_medium.png
man_small.png
woman_medium.png
women_small.png
Uploading is a different problem that boonex will have to fix. They convert every image that's uploaded which ruins both gif and png images. But uploading is not what you asked for.