While writing ParaDocs I spend a lot of time working on the icons... particularly the app icon the the document icon.
This involves editing images in multiple sizes and making sure they end up in the right place in Xcode. It's tedious.
In the end, not finding something I liked to do the job on the appstore, I wrote my own little utility to generate images of the appropriate size and export them to Xcode.
It's called McIcon. Get it free from the Mac AppStore.