Decor8 lets you ditch the stock backgrounds for the Start screen and replace them with images you've downloaded, including high-resolution photos from the web. You can even add custom effects to the background, such as blurring and fading. Decor8 also lets you select a photo of images and rotate the Start screen background periodically. Additionally, the app lets you choose how many columns and rows of tiles are on the Start screen, and Decor8 also gives you the tools to design your own colour scheme for Windows so you're not stuck with the colours available in the default settings.
You can try Decor8 free for 30 days, after which it will start nagging you to buy a full licence. The app will only set you back $US5, so if you know you'll enjoy the features it offers, you may as well buy it outright. Otherwise, grab the demo and try it out first.