Reduce opacity of existing image
Preview can't do this.
ImageMagick can, and nearly anything you can imagine.
You can easily install it with MacPorts (see MacPorts Home):
port install imagemagick
Let's say your original image is porsche-911.png:
Enter in Terminal the following command:
convert porsche-911.png -alpha set -channel A -evaluate set 50% porsche-911-50%.png
which will compute from your original input porsche-911.png the output
porsche-911-50%.png:
If you want to learn more about this great free tool, read:
ImageMagick home.