Invisible pictures in system applications
The images stored in CoreServices, such as those in /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources, should be easily viewable in Finder — though, the majority of these are in the icns format. I'm also able to view the png images inside Dock.app. (Which particular images can't you view?)



But most png images on iOS are optimized for display on iOS (which involves switching some color channels and otherwise reducing the file size), so you can't view them in OS X without some preprocessing. See this blog post for more information.