Application Lost From Dock

I went to click on my iPhoto application and it wouldn't load, it had a gray question mark on it. I clicked it again and it got dragged onto finder, and when I released my mouse it made a poof sound. I remember doing this in the beginning when I got my laptop if you wanted to get rid of an application from the dock. I went to the Mac HD and went to applications and where iPhoto used to be, is now blank. I searched in Spotlight, and went to Applications<utilities.

Is there somewhere where I can re-download it from the Apple site? Or do I have to totally reformat my computer to bring it back to me. Thanks for all your help.