$watermark = $imagine->open('/my/watermark.png');
$image = $imagine->open('/path/to/image.jpg');
$size = $image->getSize();
$wSize = $watermark->getSize();
$bottomRight = new Imagine\Image\Point($size->getWidth() - $wSize->getWidth(), $size->getHeight() - $wSize->getHeight());
$image->paste($watermark, $bottomRight);