thecoalman - 11:00 pm on Jun 13, 2009 (gmt 0) Server side you can do it on with GD or Imagemagik.
Irfanview or exifer can do batch watermarking locally and a whole lot of other tasks. Only takes a few minutes to setup. The trouble with any watermark is they are easily removed or very obtrusive. If you want to make one that is hard to remove use a gradient texture fill.
Server side you can do it on with GD or Imagemagik.