Windows 8, reviewed, can feel quite different to previous versions of Windows. For example, when you double-click on a photo, it will open in the new Photos app rather than the old Photo Viewer.
You can change this just as you did in Windows 7 or Vista, though. You can pick which program opens each type of file, or you can choose which file types are associated with each program.
Sometimes you'll see a notification pop up in Windows 8 saying there are new programs which can open this type of file. If you click this, a list will appear where you can choose which program should open all files of that type.
Alternatively, the first time you open a particular type of file, you may see the list pop up asking you to choose a default program.
However, if you don't see this, simply tap the Windows key (as long as you're not already on the Start screen) and type 'default' without the quotes. This searches for matching apps, and will display Default Programs on the left. Hit Enter if it's the top result or click on it to launch the Control Panel.
It will open on the traditional desktop and display the options for default programs. Click the 'Associate a file type or protocol with a program' link.
Scroll down the list until you find the file type you're after (there's no search facility). We're choosing .jpg here.
Click Change program.. and a list will pop up as shown below. Choose the program you want from the list, bearing in mind you may have to scroll down. If you don't find the one you're after in the list, click More options and browse for the program.