I decided to try and start signing my pictures.

composite -watermark 15% -gravity southeast sign-1.png $i $i;

It needs more tweaking but that will do for now.

To convert all files in a directory:

for i in *.JPG; do
        echo -e "$i... \c";
        composite -watermark 15% -gravity southeast sign-1.png $i $i;
        echo done;
done

Oneliner:

$ for i in *.JPG; do echo -e "$i... \c"; composite -watermark 15% -gravity southeast sign-1.png $i $i; echo done; done