Or, if your server is linux, you could use 'composite' to make a watermarked version. I'd go with this approach because if you just use CSS people can download the original image anyway.
Code:
composite -gravity southeast -dissolve 15 watermark.jpg original.jpg watermarked/original.jpg
Or, you could do something like this (untested):
Code:
for file in `ls images/*.jpg`; do composite -gravity southeast -dissolve 15 watermark.jpg $file watermarked/$file; done