Download Windows 8.1 Wallpapers
MS did a good job at choosing images that look good as Start screen backgrounds (when setting the desktop wallpaper as the background for the Start screen). They are all visually clean, go well with different UI colour themes, and the content is more or less abstract (which is important because most of the image will be covered by the live tiles). Also it seems like they are made to look good on smaller screens - wallpapers in Windows 7 and earlier were made for large high res screens, with lots of small detail.
Good to see that MS didnt just go with the same old landscape/flower themes.