What should I show while the big images are downloading tiling?

What I am doing in my app is to pre-fetch the 1x images. When a user taps the scrollview with these showing, I kick off a fetch of the bigger images but let them browse the 1x images. When the 4x images arrive, then I replace the 1x images with the 4x images (something the user doesn't even see) and then enable zooming (with a visual indication).

In v1.8 and newer, you can assign plain old images to the ImageScrollView, and the user can browse those while the bigger images download.