Can you passlock specific Apps on an iPhone? I know the easiest way to secure your iPhone is by using a 4-digit-passcode to lock the entire phone. However, I was wondering if there is a way to securely passcode-protect only the Photo and Mail apps, i.e. a passcode is required every time the apps are opened? 

My iPhone is not jailbroken and I'm not planning on doing so.
Individual apps from the App Store can't be passcode protected in this way without jailbreaking which you mentioned you don't want to do. You may be able to use restrictions, but you will need to go to Settings each time to enable/disable access, and restrictions are only available for certain pre-installed apps and features.