void Graphics::setTransparentColor( Color | ColorIndex c )
Graphics::setTransparentColor will set the transparent color, so which color will act transparent when using Graphics::drawImage.
By default, Images don't have a transparent color.
The transparent color is set per Image, so you can have different transparent colors set on different images.
After you set a transparent color, you can clear is with clearTransparentColor.
It's advised to use this function in setup().