Basically, an icon that sends the message across of the functionalities. Icon should look sophisticated and at the same time, cute (i mean, afterall, this is about attracting the younger aged market that wants to sent cute icons, stickers to their friends. Some japanese style element is required. Have a look at similar apps on the Apple Appstore, to get some ideas. I do not need to have the app name shown on the icon, so I won't provide the appname.
Of course, you should understand the dimension requirements (pixel sizes) of iPhone app icons.
50X50, 58X58, 72X72, 96X96, 114X114, 144X144, 512X512, 1024X1024