Some smart apps are pushing the limits of your iPhone
camera these days. Take a look at Action Shot. This free app creates an effect that would take me hours to bang out on my Mac.
Here’s how it works: You shoot a brief video of some action. The app presents you with a handful of frames. You drag to shade over your action-inducing subject in each frame and the app combines them into a still. A still with your subject displayed multiple times as it progresses across the frame.
Great sample here from Apple’N'Apps.
Even better, you don’t need a tripod. It would help, but the app can do its job if your hand is somewhat steady.  For best results hold the phone as steady as possible and let the subject move across the frame – rather than tracking the subject.