Dainank 

You can access the Microsoft To Do backgrounds in the file path below:

C:\Program Files\WindowsApps\Microsoft.Todos_2.114.7122.0_x64__8wekyb3d8bbwe\Assets\Themes\backgrounds

If this works, you should still read the last bit of this answer to make sure that you save the right file, and that you save it correctly.

This shows where the files are stored, however the exact file path may be different for you. In that case, you can open file explorer into your main drive, and open the 'Program Files' folder. You will then have to scroll down to the bottom of the list of folders and select the 'WindowsApps' folder. This folder may be hidden, so if you cannot see it, you can copy this file path into the address bar in File Explorer:

C:\Program Files\WindowsApps

If that does not work, then you can follow what has been done in the image below:

You want to have the tick next to hidden items. Once that is complete, you should be able to see the 'WindowsApps' folder, and you can open that.

Once in that folder, you want to look for another folder named in a way similar to the following name:

'Microsoft.Todos_2.114.7122.0_x64__8wekyb3d8bbwe'

You can then open that folder, and then open the 'Assets' folder, then open the 'Themes' folder. Once in this folder, you see another folder called 'backgrounds', and within that folder, you should find a list of folders with various names as you can see below:

These folders contain the images used for each background in Microsoft To Do.

Within these folders, you will want to save the image that is named with the image resolution, not the one called 'thumb':

Ensure that when you save this file, you do not remove it from this location, and that you copy it into whichever folder that you store your backgrounds in, otherwise you will not be able to use this background in Microsoft To Do. Your system should block this anyway, but if it doesn't, make sure that the background stays there.

Hope this helps.