Hello i would like to share this with you, there is a lot of tutrials writen on net for watermarking image, but i dident find example for silverstripe:

create ImageDecorator.php (you can named as you want) put in this code

<?php
class ImageDecorator extends DataObjectDecorator {

    /**
     * Put on uploaded images watermark
     */
    function onAfterWrite(){
        parent::onAfterWrite();

        //absolute path to watermark in mycase is watermark located in http://localhost/ak/mysite/code/images/watermark.png
        $watermark = imagecreatefrompng(Director::absoluteBaseURL().'mysite/code/images/watermark.png');

        //get watermark width
        $watermark_width = imagesx($watermark);

        //get watermark height
        $watermark_height = imagesy($watermark);

        //imagecreatetruecolor() returns an image identifier representing a black image of the specified size. http://si.php.net/manual/en/function.imagecreatetruecolor.php
        $image = imagecreatetruecolor($watermark_width, $watermark_height);

        //imagecreatefromjpeg() returns an image identifier representing the image obtained from the given filename. http://si.php.net/manual/en/function.imagecreatefromjpeg.php
        $image = imagecreatefromjpeg(str_replace('/','\\',Director::baseFolder().'/'.$this->owner->Filename));

        //array getimagesize  ( string $filename  [, array &$imageinfo  ] ) http://si.php.net/manual/en/function.getimagesize.php
        $size = getimagesize(str_replace('/','\\',Director::baseFolder().'/'.$this->owner->Filename));

        //x-coordinate of destination point.
        $dest_x = $size[0] - $watermark_width - 5;

        //y-coordinate of destination point.
        $dest_y = $size[1] - $watermark_height - 5;

        //imagecopymerge — Copy and merge part of an image http://si.php.net/manual/en/function.imagecopymerge.php
        imagecopymerge($image, $watermark, $dest_x, $dest_y, 0, 0, $watermark_width, $watermark_height, 100);

        //imagejpeg — Output image to browser or file , I override uploaded image with the watermarked image  http://si.php.net/manual/en/function.imagejpeg.php
        imagejpeg($image,str_replace('/','\\',Director::baseFolder().'/'.$this->owner->Filename));

        //imagedestroy — Destroy an image http://si.php.net/manual/en/function.imagedestroy.php
        imagedestroy($image);
        imagedestroy($watermark);
    }
}
?>

in _config.php insert this code

Object::add_extension('Image','ImageDecorator');

that's all, it's work fine for me

Need help with this - does not work says:

"There was a problem with the upload"

try again.

Any hints why ? 2.4.5