I downloaded the icons and they do have the transparent background layer.
I chose the sunny one that's showing right now.
I can't get the transparency working with either the original icon or this one.
The config.txt only has 3 settings that affect this property:
$trans_color = imagecolorallocate($img,250,250,250);
$trans_color = imagecolortransparent($img,$trans_color);
$anti_alias = 'on';
If I set the color allocate setting to 0,0,0 (black) then the background is black. If I change it to 255,255,255 (white) then the background is white. Any other color setting has no effect.
Setting the antialias to either yes or no has no effect.
It's going to require more code horsepower than I have.