You can try something like this.... This is in C# though, I'm sure the main methods that make this work aren't supported through javascript, but this might help you see how it is done in other languages.
Image img = Image.FromFile ( "...", true );
if ( (img.Flags & 0x2) != 0)
{
bool ContainsTransparent(Bitmap image)
{
for (int y = 0; y < image.Height; ++y)
{
for (int x = 0; x < image.Width; ++x)
{
if (image.getPixel(x, y).A != 255)
{
return true;
}
}
}
return false;
}
}
