You can make four icons (as of today) for your app and they can all have a different look – not necessarily based on the 512×512 image.
- corner radius for the 512×512 icon = 80 (iTunesArtwork)
- corner radius for the 1024×1024 icon = 180 (iTunesArtwork Retina)
- corner radius for the 57×57 icon = 9 (iPhone/iPod Touch)
- corner radius for the 114×114 icon = 18 (iPhone/iPod Touch Retina)
- corner radius for the 72×72 icon = 11 (iPad)
- corner radius for the 144×144 icon = 23 (iPad Retina)
If you do create a set of custom icons, you can set the UIPrerenderedIcon option to true in your info.plist file and it will not add the gloss effect but it will place a black background under it and still round the image corners with these corner radii so if the corner radius on any of the icons is greater then it will show black around the edges/corners.
Edit: See comment from @devin-g-rhode and you can see that any future icon sizes should have a 1:6.4 ratio of corner radius to icon size. There is also a very good answer from https://stackoverflow.com/a/29550364/396005 which has the location of image mask files used in the SDK for rounding icon corners
To add a retina-compatible file, use the same file name and add ‘@2x’. So if I had a file for my 72×72 icon named icon.png, I would also add a 114×114 PNG file named icon@2x.png to the project/target and Xcode would automatically use that as the icon on a retina display. You can see this in action on the Summary page of the application target if you’ve done it right. The same works for your launch images. Use launch.png at 320×480 and launch@2x.png at 640×960.