Either your icons are in the wrong directory or your config.xml points to the wrong path, I assume.
Since I can't see your source, I would be guessing. Therefore, you better check first: does your config.xml show something like this?:
https://github.com/phonegap/phonegap-...
(I would add 76x76, 152x152 and 120x120, though)
Make sure you have .png images of exactly the correct size, and make sure the path mentioned in your config.xml equals the directory path of the images.