The post Make a Windows Store app display icons at appropriate sizes in C# shows how to give a Windows Store app the images it need in appropriate sizes. The process is similar for a Windows Phone app, except the sizes of the images is different.
See the previous post for information about how to set the images. The following table lists the image sizes with the required sizes highlighted in red.
As is the case with Windows Store apps, the images must have exactly the correct sizes for a Windows Phone app. The application will resize a required image within a category if it needs another size in that category.