Edition image bug

`Images` are being saved without any `ImageData`, which is resulting in exceptions. I suspect this due to a browser quirk that means the nested image data attributes are not being posted when they are blank, and because the `Image` class does not have any validations on `ImageData` being present, all that's required is some alt text and the image will validate. This adds the missing validation and cleans up the crumby images that have no file.