Hi - there doesn't seem to be a way of deleting an image from a background or an article/collection.
Any chance you can add a tick box to 'Remove Image'
I'll let the PMs address the request.
In the meantime, I keep an invisible png file for just this situation. Load it to wipe out the previous image.
I use a 100x100 invisible png, I find it's easier to find than a 1x1 invisible png.