In order to take advantage of the different screen resolutions and pixel densities that your Windows 8 and windows Phone apps will be displayed at, you’ll need to output some of your apps’ bitmap assets at different sizes.
Why do you have create so many app icons?
This is simply related to the different in screen sizes, resolutions and pixel densities across devices, read these guidelines for scaling to pixel density.
Getting Started building Icons
To create all the required images for the current Windows Store application requires you to generate 44 images at specific sizes and which need to follow a defined naming convention for Window’s resource loader to supply the right image for the right device.
There a tool
Check out the Windows Store Image Generator.
The Window Store Image Generator simply allows you to upload one image which it uses to automatically generate the 44 images your project requires. You can then download them in one ZIP file and drag and drop into your VS project.
This tool saves at least a couple of hours of pixel resizing!
The tool suggested here isn't of much use for Windows 8.1 apps because quite a few resolutions won't get generated.
Try this one instead: