If it's a new image that you haven't posted already, but it doesn't look good to you and want to change it, just copy all the description and keywords to a text sheet on your computer. When you upload the new image, you paste of course all those in the respective fields. Not sure if this was your question or not...
If it's an old post, nothing will change when updating the image to a new one, except for the products that have to be redone