Invisible watermarking / GD2

Hi I use GD for stamping images with a visible watermark - in this case, a text string and a tracking number appended to the bottom of each image. However, I'd like to embed tracking information (I sometimes see my pics crop up on mailing lists... And they're the ones I see copied!) into the picture. Digimarc have a nifty looking copyrighting setup, but with a price-tag of over three grand for an automated system, it's not affordable. Does anyone know how to have PHP /GD invisibly watermark images? The info on the PHP site's GD info pages suggests this can't be done. I'd be interested to hear otherwise though.