An answer is here: http://makeappicon.com/ios7icon#
It has a good description of what is required, and a web app to generate them.
You have to give them an email to get the icons they generate, but it seemed worth it to me.
Unfortunately, I get:
The following build commands failed:
CopyPNGFile Debug-iphoneos/Leap.app/Iconfirstname.lastname@example.org /Users/mgregory/Documents/Leap/Leap/ios/Iconemail@example.com
CopyPNGFile Debug-iphoneos/Leap.app/Icon-Small-40.png /Users/mgregory/Documents/Leap/Leap/ios/Icon-Small-40.png
CopyPNGFile Debug-iphoneos/Leap.app/Icon-Smallfirstname.lastname@example.org /Users/mgregory/Documents/Leap/Leap/ios/Icon-Smallemail@example.com
IE the list these guys give doesn’t match what something seems to want. I still haven’t worked out what is determining what is needed