Iconfit - Simple Resizer
By EventVoiceMedia GmbH
Open the Mac App Store to buy and download apps.
Just drag 'n drop your Images / Icons on the App Window, click the "Save Icons to Photo-Folder" Button and you will receive a iOS compatible Icon in Default resolution (57x57 px) and one in Retina resolution (114x114 px).
Rudimentary but dependable
While developing iOS apps, I like to change the icon to random images just to keep things interesting.. this is an easy way.