Windows 10 Technical Preview

Builds 9xxx to 9xxx
At this point in development, 10's wallpapers are identical to 8.1's, except that img0 and img1 have swapped numbers, resulting in 8.1's img1 becoming the default wallpaper.

Builds 9xxx to 10xxx
All of the Lines and colors wallpapers leftover from 8.1 have been removed and replaced with a completely different theme, consisting of stock photos mostly licensed from Getty Images, most of which use rights-managed licenses. None of these appear in the final release.

Builds 9xxx to 9xxx
At this point in development, 10's lock screen wallpapers are identical to 8.1.

Builds 9xxx to 10xxx
All of 8.1's lock screen wallpapers have been replaced with new ones; much like the desktop wallpapers, these are also mostly stock photos licensed from Getty, although most of them use a royalty free license instead.