Avoid using any app icon including transparency as it will result with icon having a black backgroud which is ugly. The documentation actually states this here https://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/AppIcons.html
Avoid transparency. An app icon should be opaque. If the icon’s boundaries are smaller than the recommended sizes—or you use transparency to create “see-through” areas—the resulting icon can appear to float on a black background, which tends to look especially unattractive on the beautiful wallpapers that users choose.