Why can't I "slide to unlock" picture messages?

I'm not sure why this bugs me, but...

I just recently discovered that I can swipe a text message on my iPhone 5's lock screen and it will bring up my "Enter Passcode" screen. After I enter in the code, it takes me right to the text message.

However, when I get a picture message, it will NOT let me swipe the photo to bring up the "Enter Passcode" screen, and therefore I cannot go directly to the picture message like I can with a regular text.

Anybody know why this happens and if there's any way to fix it?

Thanks!

PS: I'm on iOS 6.1.4 right now.