I'm looking for iCoCo to be used with El Capitan.
This software won't be available for El Capitan in the near future because it's discontinued. The old website is available only through Archive.ORG website. Therefore, I recommend using another software called Img2icns and it's also free and supports EL Capitan OSX releases.
Use the following links or search it through App Store in your Mac: https://itunes.apple.com/us/app/image2icon-make-your-own-icons/id992115977?ls=1&mt=12