5 second delay caused by itok
If you can, look instead at serving the image as a public file, so that your webserver can return it directly without loading Drupal at all for that request. Instead of seeing system/files in the path, you'd see a path to the actual image file on your server - eg sites/default/files/.

Review whether it's necessary to serve images using the Private download method, which introduces a Drupal bootstrap and access controls on all static file requests. The default for this is configurable at admin/config/media/file-system.

It may be you're seeing a delay across all requests handled by Drupal, and have noticed it because you'd expect serving a static file to be quicker than that? Note that requests for static files (eg your logo) are not similarly affected, but for me I see ~6s response time checking the more basic pages on your site (eg curl --head against your donate page).

Is your server overloaded, or do you have modules enabled which might be performing time-consuming operations (eg network activity, DNS lookup, ...) for every request? 

A patch in d.o#2089789 gives you a means to remove the itok token if you do need to serve that image after checking Drupal's access controls.

The reasons for introducing itok are discussed in the Drupal 7.20 release notes.