Η Microsoft released a set of official Wallpapers. The set includes 19 wallpapers and each of them comes in different sizes.
The new Wallpapers are available on the Microsoft Design page. Scroll down until you find the image you like.
The site provides additional details for each image. On the left, you will see the purpose of image curation, while on the right, there are some information for its curators.
To download an individual image, click on button bottom right. A ZIP file will be downloaded, containing the image you selected. 
The sizes are different, as the images are aimed at many different Android, Desktop, Surface Duo and iPhone devices. Choose the size of the image that fits your desktop resolution.
Microsoft Design has 19 wallpapers that include:
- Icon System Celebration
- Microsoft 365 Vision
- Office Icons
- Pride
- Surface Duo
- Surface go 2
- 3 Laptop Surface
- Surface ProX
- 3 Surface Book
- Surface Pro 7
It is also possible to download the entire set. At the bottom of the page, click Download All. You will download a 170MB ZIP file with all the images and all their sizes.
https://wallpapers.microsoft.design/
