There are a bunch of image sizes associated with Windows 8 apps. Your logo is 150 x 150 pixels. The small logo is 30 x 30. The splash screen is 620 x 300. And the store logo is 50 x 50. You should also have some promotional images. At the very least you should provide one that is 414 x 180.
Your app has to pass tests from the Windows App Certification Kit (ACK). This tool laucnhes your app. This must be the release version of your app. I use version 2.2 of the ACK to do my business. Don't worry too much. Visual Studio will guide you through running it before uploading to the Windows Store.
You must register as a Windows Developer to upload your apps to the Windows Store. Normally this costs $49 for an individual account. A corporate account will oost you $99. I think these are the yearly fees. I hear you can get a free account if you are a student and you can prove it. At least you used to be able to get a free student account.
Watch out. A human reviews your app for inclusion in the Windows 8 Store. You app has to "provide real value" or it could get rejected. At least one of mine got rejected. I gave up on that app. Other more worthwhile apps that initially got rejected eventually got through. Some I am still working on.
