( resource image, bool blendmode )
imagealphablending() allows for two different
modes of drawing on truecolor images. In blending mode, the
alpha channel component of the color supplied to all drawing function,
such as imagesetpixel() determines how much of the
underlying color should be allowed to shine through. As a result, gd
automatically blends the existing color at that point with the drawing color,
and stores the result in the image. The resulting pixel is opaque. In
non-blending mode, the drawing color is copied literally with its alpha channel
information, replacing the destination pixel. Blending mode is not available
when drawing on palette images.
If blendmode is TRUE, then blending mode is enabled, otherwise
Returns TRUE on success or FALSE on failure.
Note: This function requires GD 2.0.1 or later.