Windows 10 - where are custom themes' pictures stored:
 
"Go to File Explorer:

%LocalAppData%\Microsoft\Windows\Themes

Which will take you to wherever your default themes are saved"

Date of note creation: June 19th, 2019

from: https://answers.microsoft.com/en-us/windows/forum/windows_10-start/windows-10-where-are-custom-themes-pictures-stored/82eab288-9aaa-4723-9f43-859eafb603a3