Quick and dirty way to add a water mark to all images in a folder.

watermark.sh

#!/bin/bash
WATERMARK="$1" #1st param image to be the water mark

#2nd param is path to folder you want all jpg, jpeg or png to
#have the water mark added to
for image in $2/*{.jpg,.jpeg,.png}
do
    #gravity is the position the water mark will go
    #50 is the opacity of the image
    #the second $image is the output file, currently
    #this will overwrite the input file, feel free to change to something else
    #or place in a different directory
    composite -gravity southeast -dissolve 50 $WATERMARK "$image" "$image" &> /dev/null
done
exit 0