l2ob, welcome here and welcome to the Mac platform.
To get that icon back, go to your Applications folder, grab the Photo Booth.app icon and drag it to the Dock with the other aliases (= Mac jargon meaning the same as "shortcuts" in the Windows jargon) already there and simply place it where you want it to be.
Don't drop that icon on the way to the Dock though 'cause that will take out Photo Booth out of the Applications folder. If this happens, it's no biggie though: you simply need to drag it back in the Applications folder.