I usually use my iPhone to take pictures for my Website and then upload them via the Dropbox app so they appear on my computer.  This is a nice feature, but the photos are way too large to use on a Website.  Instead of opening each one in Preview, scaling it down and saving it.  I created a Hazel workflow that does this for me automatically.  It then copies the original file to the Photos folder.

You will notice I have workflow for the LaunchAgent/LaunchDaemon folders.  Those are for my roll-your-own malware detection, which works nicely!

The script is pretty straightforward.  I just run two commands–one to copy the file and one to scale it down.

cp "$1" ~/Dropbox/Photos/ sips -Z 800 "$1"

Then, I have a rule that looks for any jpg  or png  files and runs an embedded script on the results.  The sips  (scriptable image processing system) command is built-in to OS X, but you could also use something like ImageMagick if you wanted.  Personally, I tend to use native commands before installing additional software.