I just had a look at
the white queen, here is a sample code, not so hard in fact
Code:
def showCard= { cards ->
def baseImage = java.awt.Toolkit.getDefaultToolkit().getImage(
"images/xxx/base.png")
def width = baseImage.getWidth()
def height = baseImage.getHeight()
def image = new java.awt.image.BufferedImage(width, height, java.awt.image.BufferedImage.TYPE_4BYTE_ABGR)
def graphics = image.getGraphics()
graphics.drawImage(baseImage, 0, 0, null)
def thisImage = javax.imageio.ImageIO.read(new File("images/whitequeen/"+card+".png"))
graphics.drawImage(thisImage, 100, 100, null)
setImage(image, false)
}