Actually you could just download the encrypted images without the unique key( for every image ) and then get the key on demand. Load the image into ram, decrypt, show the image and then erase everything.

That would be more secure, but you still need to wait for the key to be downloaded on-demand - you don't exactly get butter-smooth scrolling that way.

(I like security.)

And you would have to be connected to the internet. That is a problem. The delay could be hidden behind a ui.

Snapchat doesn't work particularly well offline...