In a file's context menu, there's an
Open with submenu. From there, select
Choose default program.... A metro-style dialog will pop up in the center of your screen. For pictures, the desktop program is "Windows Photo Viewer". Unfortunately, you'll have to repeat this procedure for the various picture file types (BMP, JPEG, PNG, ...).
For videos, you can use Windows Media Player or any other player of your choice, e.g. VLC Player. Same goes for music, the classic option is Windows Media Player. And yet again you'll have to repeat the procedure for all video and audio file types.
Oh yeah, I almost forgot: In the Control Panel, there's the "Set Default Programs" mechanism. Using this (and a compatible program) the process is a little less time-consuming. Simply select the Windows Photo Viewer and press "Set this program as default". Repeat this process for all associations you want to change.