Not trying to troll, but doesn't this all boil down to DRM? Why not simply create a DRM-enabled jpeg format that mobile devices support and be done with it?
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How would a different (DRM'd) format do anything to prevent screenshots?
DRM is a function of the viewer, not the format, and your ability to create effective DRM on iOS is limited by the lack of a way to diable screenshots (unless it's a corporate-owned device with systemwide policies).
It is also limited to some extent by the lack of remote attestation of non-jailbroken-ness using some sort of hardware crypto module.