If you need to make iOS app icon assets from a photo, take a look at the Make App Icon website and API. They will let you upload an image from a web browser (in exchange for an email) and then email you the assets you can just drop into your XCode project.
However, for more advanced users, just use the command line and their API.
curl --upload-file Photo.jpg http://makeappicon.com/make/ios > Photo.zip
See docs for more info