Here's a simple program that adds your watermark to most image formats, plus PDF. Free trial version, $10 for the licensed app.
http://quoteunquoteapps.com/bronson/
Photoshop would be better if you need to watermark a large number of images, since you can create a batch process that adds a watermark layer.
http://quoteunquoteapps.com/bronson/
Photoshop would be better if you need to watermark a large number of images, since you can create a batch process that adds a watermark layer.