Fix a bug related to editing an existing image.

SiteImage uploads have always allowed editing the image, without actually replacing the image.

I believe this is primarily to allow editing other fields, such as title if available. Despite the base Site package upload page having no further fields beyond the upload widgets, I've kept this behaviour for easier subclassing and use in existing pages. What this does mean is that we shouldn't replace teh existing object, unless new image is uploaded.