These files are not views, so they cannot be overridden that way.

There are a number of places depending on which files you want to overwrite - sometimes it's in a plugin hook that returns a default url for an image, sometimes it's in a view where you can override the url.  I there was some discussion about making them all part of the views system but that would further tax the server.

Create a _graphics folder within your own theme where you place your replacement images and change the path to the corresponding images within the CSS files you are already overriding.

Yea, I'd prefer we move these into the views system. We have plans to support skipping PHP entirely for simplecache files if sites really need that extra performance boost.