Varnish provides a reverse proxy cache and has been an invaluable tool to put in front of dynamic image processing servers so that large groups of images can be resized and watermarked on the fly.

The Varnish web application accelerator is something that has allowed me to handle static content behind pound without hitting Apache or any of my other application servers.

In my Pound configuration I redirect all static content extensions to a varnish server, which then will forward all cache misses to the appropriate back-end server.

By setting cache control headers, the content gets cached in varnish. I have servers that do on the fly dynamic resizing of images, and without the caching, it would easily be my highest load, but with varnish it's a non-issue.