( resource image, bool saveflag )
imagesavealpha() sets the flag to attempt to save
full alpha channel information (as opposed to single-color transparency)
when saving PNG images.
You have to unset alphablending
(imagealphablending($im, FALSE)), to use it.
Alpha channel is not supported by all browsers, if you have problem with
your browser, try to load your script with an alpha channel compliant
browser, e.g. latest Mozilla.
Note: This function requires GD 2.0.1 or later.
See also imagealphablending().