What you are looking for is something called "Billboards". If the 3D camera moves, the sprites will always face the camera, always displaying the image. Games like Ragnarok Online do this. Normally what you do is just draw a quad and transform it so it's always facing the camera.
I'd link a tutorial but it really depends on what technology you are using for the iPhone app. Is it cocos3d? Unity? OpenGL?
Try looking for