Android: Selectively securing apps can provide you with an extra layer of security. ZDlock not only allows you to protect apps with a PIN or pattern, it can even fake a crashed app dialog to discourage snoopers.
Like most security, it won't keep out the most determined thieves. However, if your nosy friend tries to open up your Gallery and sees a consistent "error message", they may give up. Even if they do know how to bypass the error message, they'll still need to enter your PIN or pattern to continue.