I have a freshly created Cordova project with the following config.xml setup (used the instructions from http://docs.phonegap.com/en/edge/config_ref_images.md.h
If you are using cordova 3.5.0 they have updated the docs. In older versions i've always had to replace the icons manually in the project but the latest version of cordova is working fine.
http://cordova.apache.org/docs/en/3.5.0/config_ref_images.md.html#Icons%20and%20Splash%20Screens
As you can see here https://github.com/phonegap/phonegap-cli/issues/58 it's been a common problem. So if you are using an older version of cordova i recommend to update it with the command npm update -g cordova
And after that you should update your config.xml to something like this:
As you can see the URIs are relative to the cordova project's path, not to the www folder.