Bjango in “PNG compression and iOS apps“ on their blog:

[…] if you’re spending any effort compressing or optimising your PNG images for iOS app development, you’re wasting your time.

And then later:

In an effort to dramatically increase drawing performance of iOS apps, Xcode re-compresses PNG files as it builds. It premultiplies the alpha channel and byte swaps the red, green and blue channels to be sequenced blue, green and red. The result is optimised for iOS’s purpose, but as a side effect, ImageOptim’s work gets undone […]

I use ImageOptim all the time (it’s a great app), but I had no idea it’s meaningless when working with iOS.