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
Try following https://www.npmjs.org/package/cordova-gen-icon
Example:
$ cordova create hello com.example.hello
Creating a new cordova project with name "HelloCordova" and id "com.example.hello" at location "hello"
$ cd hello
$ cordova platform add ios
Creating ios project...
Preparing ios project
$ cordova-gen-icon
Generate cordova icons with
project: .
icon : ./www/img/logo.png
target :
generate iOS icons
Success generate icon set