Notice how some of the images look very odd. They look very much like they've been created as white images with a transparent background on the intention/assumption that they will always be displayed on a black background. Yes, that assumption was wrong.
Remember, Windows Phone 7 is a platform where quality is essential. How you do something is more important than what you do.
The marketplace is like your store front and the icon is like your packaging. Would you think twice about something with poorly printed packaging? Would it make you question the attention to detail paid in the rest of the production process? And, would you even reconsider buying from a store where products were poorly packaged?
Please don't make this same mistake with your apps.
Bad apps don't just hurt the developers, they hurt all developers as they hurt the platform. If it was up to me apps with images like that shown above would have failed the submission process.