Occasionally when uploading your own photos, or artwork, or even if you run a site that hosts images and you wish to mark them, watermarking can be a useful technique. The aim is to simply overlay an unobtrusive graphic over your source image, I find white text, with a black shadow seems to work the best on both light and dark source images.
It's rather easy in PHP and GD2, and as a bonus we can use a PNG image with alpha transparancy as the "watermark" image, which means proper antialiasing on text, and dropshadows are possible!
You'll need a source JPG image, and also a small watermark image - the script below is well commented so you can see how each step works, simply change the first two variables to point to your image files on the server.
An example using my setup below can be found here (water mark is in the bottom right corner).
/* setup the source file and watermark */
$sourceImg = 'beach_large.jpg';
$watermarkImg = 'watermark.png';
/* create source file with alphablending */
$image = imagecreatefromjpeg($sourceImg);
/* create watermark holder */
$watermark = imagecreatefrompng($watermarkImg);
/* get image and watermark dimensions */
$width = imagesx($image);
$height = imagesy($image);
$wmWidth = imagesx($watermark);
$wmHeight = imagesy($watermark);
/* caclulate where to place watermark */
$xPos = $width - $wmWidth;
$yPos = $height - $wmHeight;
/* Copy the watermark to the top right of original image */
imagecopy($image, $watermark, $xPos, $yPos, 0, 0, $wmWidth, $wmHeight);
/* output image directly to browser */