IPhone :: Accidentally Deleted Photos App Icon From It

I've read the previous threads on this topic but none quite fit the bill. I need to re-install the Photos icon, which has disappeared.

This happened while I was holding my phone but not paying attention; when I looked again the icons were all shaking. I don't know how the icon disappeared because the factory defaults don't offer a delete option, but the icon definately disappeared at that time.The app itself is still there and, because it was running when I deleted the icon, I can currently still access it by double clicking the main button, where app (and icon) is still accessible there at the bottom of the screen. But this is not convenient for general usage, and if I hard close the app from there then I will have no way to open the Photos app.

I can't re-download the app because it is a factory default app and not available anywhere (and as such shouldnt be deletable!).The icon isnt hiding anywhere amongst my subfolders, and I've checked for restrictions but there arent any.Restetting the home screen layout to factory defaults presumably means I will lose all the other app icons I have, and their sub folders etc, which I dont want to do either.I have now backed up my phone and all my photos etc, but if I restore my phone from a previous backup, I will lose the data and messages I've acquired isince then.All I want to do is re-install the icon and make sure it links to the photos app that is still installed - surley that can't be so hard?

Info:iPhone 4S, iOS 5.1, Sony VAIO; Windows 7