Something that happened to almost every admin in my organisation at least once is, that we delete an image from Contentful, instead of deleting the connection of the image to a content object.
The correct way of deleting the connection when modifying a content object is clicking at the three dots at the top right and select delete (which only means that the connection between image and the content object is deleted).
The more intuitive way (but the wrong way) is to click at the image and modify it here. This deletes the media of the old image and as far as I know there is no easy way to undo the exchange of images.