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