Depends on your application. If this is going on a web site then I would say look into using PHP GD and drawing a water mark over each image. Another solution to make a transparent image overlay on top of the existing image then display that image.
watermark factory Download it here: http://www.watermarkfactory.com/watermark-factory-features.htm click where it says PROCEED NOW under the title "Protect your copyrights using visible watermarks"
this says Elements but it works with PS too http://graphicssoft.about.com/cs/photoshopelements/ht/psewatermark.htm the beauty is that once you create the action you can batch X number of images with the WM