From Foldr Settings >> Customise, you can upload a custom image file that will replace the default wallpaper on the web app login screen. The wallpaper will be sized to fill the user’s web browser window so an appropriately sized image should be provided.
Please note that the file must be provided in PNG or JPEG format and should be 1.5MB size or less. For reference, the sample v4 background image is 1800 x 1200 pixels.