Something about photos

[StandardUser]

Ive been browsing my photo gallery and suddenly i pressed some key and instead of document names(in the folder), the document names became photos(as in, i can see the photo without opening it).

What is the hotkey i pressed, how i change it, and where can i assign custom keys for these kind of actions?

[StealthFox]

Windows 7: Right click>View> and select your desired option
Windows XP: I can't remeber if right click does it too, but click on view in the toolbar and select your desired option.