Backgrounds

Has anybody worked out how to add custom backgrounds programmatically?  We will need to push our corp imagery to make it available for all staff.  Previously this was done by GPO putting images into %APPDATA%\Microsoft\Teams\Backgrounds.  The new location seems to be %LOCALAPPDATA%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams\Backgrounds\Uploads - however manual copy of images to this location, even if you include the image and thumb from the old location don't work.    When using the GUI to add images, they are put into the uploads folder with a randomised name and auto generated thumb.   I also tried copying these to a different users machine same location but it doesn't recognise them, so they must be indexed somehow elsewhere?