twzWatermark is a PHP class that watermarks a PNG, JPEG or GIF image. The watermark can be any custom text, or another image (usually a PNG with transparency).
The resulting image can be saved as a file, displayed in the browser, or returned as a PHP image resource for further processing. A watermarked image can be created with just 4 lines of code, eg
$wm = new twzWatermark('myphoto.jpg', 'mymark.png');
$wm->SetPosition('br'); // br = bottom right
The download includes some examples, and an opentype font for adding text watermarks.