Do you want your own website to promote your app? If so, check out mywp7app.com, a site specifically for Windows Phone developers!
For only $15 a year, you can get a custom made website just for your app. On it, you can post unlimited screenshots, videos, QR codes, beta XAP files, change logs, detailed features list, help pages, etc!
If you know HTML, you can also create your own site by hand. Plus, you get access to a FTP server.
Check out mywp7app.com for more details!