Applying CSS directly to an image is not possible, as CSS is a styling language and doesn't work on images. However, you can dynamically create a new image with an overlay using PHP.
By loading a stamp image and the photo you want to watermark, you can position the stamp onto the photo and serve it as a new image with the watermark applied.
<?php
// Load the stamp and the photo to apply the watermark to
$stamp = imagecreatefrompng('stamp.png');
$im = imagecreatefromjpeg('photo.jpeg');
// Set the margins for the stamp and get the height/width of the stamp image
$marge_right = 10;
$marge_bottom = 10;
$sx = imagesx($stamp);
$sy = imagesy($stamp);
// Copy the stamp image onto our photo using the margin offsets and the photo
// width to calculate positioning of the stamp.
imagecopy($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp));
// Output and free memory
header('Content-type: image/png');
imagepng($im);
imagedestroy($im);
?>
Source: http://php.net/manual/en/image.examples-watermark.php