Okay - there seems to be a bit of a bug with the Image upload process - for backgrounds. Once you upload the new image and watch it load in the portal, you click DONE, and the collection you were editing doesn't say 'Out of date'. Stays on 'published'. I've signed out, and also refreshed the page - and also gone back through the menu and back in to the article. No joy!
Agree. When updating an image that is the same exact pixel size, you need to 'trick' the system and upload a different image, click Done. Then go back to the image you want and reload it. The portal will accept the new image.