One of the prerequisites within Xcode is setting the 'Default' and 'Default@2x' images for when the iPhone app loads up.
Is it all possible to have a number of Default screens made, inserted into the project in png format and each time the app is loaded, choose a random default screen to use?
You cannot change Default.png. Once the app is shipped - it's "set in stone". Until the next release, that is. The reason is simple (and same why you can't change apps icon). App's bundle is read-only. It is made read only because it is signed by you and by apple. Modifying the contents of the bundle would invalidate the signature.