MacOS Folder Action to watermark images using ImageMagick 6 for posting on Facebook

I wanted to prepare images to be posted on Facebook which means I needed to resize them to 1200x600 pixels and overlay a watermark.  I also want to keep a local copy of the image I post, so I decided to automate the image manipulations with a folder action and GraphicsMagick 1.3, which is available in MacPorts.  This is tested in Mojave and Catalina, but should work on most version of MacOS and OS X

NOTE: My solution modifies the image that is saved in the folder and makes no backup copies. It also creates a temporary file called "wm" in the watched folder, then deletes it.

Folder actions are performed on all new files that are put into a certain folder.  I created a new folder action in Automator with one action called "Run Shell Script".  In the top-right corner, I told Automator to pass the files as arguments to the script.  The script itself is shown below:

for f in "$@"
do
 dname=$(dirname "$f")
 /opt/local/bin/gm mogrify "$f" -resize 1200x630
 wmsize=$(/opt/local/bin/gm identify "$f" -format %wx%h)
 /opt/local/bin/gm convert "/Users/josh/Pictures/test-watermark.png" -resize $wmsize "$dname\wm"
 /opt/local/bin/gm composite -gravity center "$dname\wm" "$f" -dissolve 200 "$f"
 rm "$dname\wm"
done

So the workflow to make new image for a facebook post is simple:
1. Drag the desired image from Photos, Aperture, or anywhere else and drop it into the watched folder in Finder.

2. Wait a few seconds for the folder action to run.

3. Drag the watermarked file into the facebook post in Safari.

See below for a screenshot of Automator showing the folder action:

Automator Action