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.