How can I disable Photos on my iPad 2?
There is no built-in support for disabling specific apps on iOS. However, there is a specific software for this (and costs $$$), but requires jailbreaking.

Currently the best way to jailbreak an iPad 2 is through JailbreakMe. It is really simple, just launch it in Safari and follow the steps.

After installing, launch Cydia. Navigate to the Cydia Store and purchase iProtect. After purchasing and activating it you should be able to "lock" your individual apps, such as the Photos application.