CustomGDBackend.php

<?php

class CustomGDBackend extends GDBackend {
    
    public function watermark() {
        $newGD = imagecreatetruecolor($width, $height);

        // Do image watermark stuff here. The current image resource is $this->gd
        imagecopyresampled($newGD, $this->gd, 0, 0, $left, $top, $width, $height, $width, $height);
        
        $output = clone $this;
		$output->setImageResource($newGD);
		return $output;
    }    
   
}

WatermarkedImage.php

<?php

class WatermarkedImage extends Image {
    
    private static $backend = "CustomGDBackend";
    
    public function generateWatermarkedImage(Image_Backend $backend, $angle, $width, $height) {
        return $backend->rotate($angle)->croppedResize($width, $height)->watermark();
    }
    
}