ImageMagick(1) can be used to put a watermark on an image easily. The command below will place a TV styled watermark on an image in the lower left corner. This is very useful to watermark images you place on the Internet.
convert pripyat.jpg ABCTV2005.png -gravity southeast -composite output.png |
The -gravity southeast parameter gives us a placement in the lower right corner. Using -gravity northwest would place it in the top left corner. Make sure the image used as a watermark is a proper size to fit as a small watermark, but this could be used in a script to watermark a folder full of images.
This is to overlay one transparent image over the top of another of the same size.
convert chernobyl-pripyat-black-ferries-wallpaper.jpg game-hud-png-4.png -gravity center -composite output.png |
This is the resulting image file.
This is also a good way to add a watermark or translucent png image over the top of another, if there is a 1920×1080 transparent image file of watermarks, and then a series of images, this would be a very good way to watermark a series of sample stock photos.