If you are using time machine. Open time machine and find photobooth and restore it.
If not: It's a pretty big hassle to do. That's why you back up your data. And the applications I use here is shareware, so it means you have to pay a fee. I believe there is a trial period so for your purposes you don't have to pay. But if there isn't a trial period, and you really want photo booth back, I would pay for the license. It really is a useful program when it comes to opening package files.
Download
pacifist. This is a shareware utility used for opening .pkg files. After you open pacifist insert your mac osx install disc and in pacifist click open apple install discs. You are going to have to wait a minute for all of them to open. After you do this go into the search bar and search for
photo booth.app. This should be a subfolder in the
Contents of Essentials.pkg file. Select the
photobooth.app file. Then click
extract to at the top and select your applications folder. After this is done go into applications and double click on photo booth and photo booth should open just like before.
Next time make sure you back up your data.