Here is a website that is free and will let you upload up to 10 images at a time and then will water mark them and zip the file for easy download. I just used the defaults but you can place any text any color and font you wish.
A bonus is that it does not rename your photo so you could copy your images folder from your website to your computer then replace those with the watermarked images then just re upload it to the web server and all your pics will be water marked.
http://watermark-images.com/Default.aspx