This isn’t so much a tip for certification, as for improving the chances that your app might be highlighted in the Store. To put it simply, if you don’t include great promotional graphics with your app submission, then the Store doesn’t have what it needs to put your app in the spotlight. Here’s the quote from the documentation, from What to include in your app’s description:
The Windows Store editorial team uses promotional images when they feature apps in the Windows Store. Providing promotional images for your app doesn’t guarantee your app will be featured. But not providing them almost guarantees that it won’t be.
For more info about creating and uploading promotional images for your app, see Promotional images.
That last link above will give you exact dimensions and other requirements for those image.