Adding a watermark using ImageMagick

First, create your watermark image.  This can be text or logo, in any format.  For best results, use either a black or white image and adjust the opacity to 50%.

Then, run the following command:

composite -gravity center wm.png image.jpg output.jpg

where wm.png is the watermark image, image.jpg is the image to watermark, and output.jpg is the output image.  The -gravity option has alternative parameters, e.g., north, south, southwest, etc. 

If you have several images to watermark, you can run the following script:

for i in *.JPG
> do composite -gravity center vis-watermark.png $i tmp.jpg;
> mv tmp.jpg $i;
> done