Free desktop wallpapers
You can find many wallpapers here for multiple resolutions. On this page you will find the ones in 2736×1824, 3440×1440 and 3840×1200 resolutions. For the 1600×1200 and 1920×1200, head over to this page.
All wallpapers are for free and can be distributed further, just link back to this site (of course only for personal use, no commercial use allowed without my permission).
3:2 Surface Pro 4, Surface Pro (2017) and Surface Pro 6 wallpapers
These are all sized exactly for the Surface Pro 4 2736×1824 screen, the Surface Pro (2017) and also the Surface Pro 6, but will look great also on all other screens with 3:2 aspect rations, like the Surface 3, Surface Pro 3, Surface Laptop and of course the Surface Book and Studio (there it will be a tiny bit stretched, but still should look great)
These are all sized for a 34inch 21:9 screen, with a resolution of 3440×1440, but will also work nicely on any smaller (and even bigger) screen with the aspect ratio of 21:9.
Dual screen wallpapers
This are all made for a setup of dual Full HD monitors 1920×1200. The files are 3840×1200 big, so they should be enough also for any other dual screen setup (if your resolutions are not bigger than Full HD).