"Back in the day" animated GIFs were ubiquitous online.

They were simple to use - just pop em' in wherever you want an animated image. 

I'd like to display an animated image within an iOS application (for example, a graphic illustrating a letter being opened from an envelope).

But, I'm not sure how to go about this. 

Obviously I don't want to use an animated GIF.

A video of some sort seems like it might be the preferred solution. But, what about transparency? Also, what about the retina display? 

Thanks.