Docs: Document resetting of images

docs/API/differences_in_mastoapi_responses.md

@@ -182,10 +182,12 @@ Additional parameters can be added to the JSON body/Form data:
 - `pleroma_settings_store` - Opaque user settings to be saved on the backend.
 - `skip_thread_containment` - if true, skip filtering out broken threads
 - `allow_following_move` - if true, allows automatically follow moved following accounts
-- `pleroma_background_image` - sets the background image of the user.
+- `pleroma_background_image` - sets the background image of the user. Can be set to "" (an empty string) to reset.
 - `discoverable` - if true, discovery of this account in search results and other services is allowed.
 - `actor_type` - the type of this account.
 
+All images (avatar, banner and background) can be reset to the default by sending an empty string ("") instead of a file.
+
 ### Pleroma Settings Store
 
 Pleroma has mechanism that allows frontends to save blobs of json for each user on the backend. This can be used to save frontend-specific settings for a user that the backend does not need to know about.