Could not find Developer Disk Image iOS 9.1 & Xcode Version 7.0.1 (7A1001)
As explained on this Apple forum , you need to update to Xcode 7.1 once you have updated your phone to iOS 9.1.
UPDATE FOR iOS 9.2
With the most recent update to iOS 9.2, the same problem will occur with Xcode at 7.1; you need to update to Xcode 7.2
It seems that Apple is forcing developers to stay up-to-date with both Xcode and iOS. If you see "Could not find Developer Disk Image" in your Xcode, then your best bet to solve it is to update both your Xcode AND iOS to the most recent version.
UPDATE FOR iOS 9.3
It seems that Apple has thrown another monkeywrench into updating your Xcode. Now, not only does your Xcode have to be updated, but your OSX needs to be on El Capitan. In fact, the store will not even suggest the update to Xcode 7.3 until you change the OSX.
for iOS 9.2 and XCode 7.2
I believe this is an app store bug. Because the app store says my Xcode is update to date (7.2) but then when I go to "About" in Xcode menu it still says I'm on 7.1. I manually download Xcode 7.2 outside of Appstore and it worked
Link to manually download 7.2https://developer.apple.com/downloads/