I find it hard to believe that every developer creates their icons manually in various different ways but if that is the case, that is fine. I was wonder what the standard was for creating iOS icons (home screen icons, etc.)
Apple do have some guidelines for creating icon of app. Via:
Doing some search, you'll find a lot of article talking about icon making experience. Such as:
http://eddit.com/notebook/archive/iphone_app_icon_template/
http://pixelresort.com/blog/iphone-app-icon-design-best-practises/
And here's a tutorial on making icon:
I hope those links help.