The image field shown when a user is not logged into the site or does not have permission to upload to the media library uses the standard browser file field. Unfortunately, there isn’t really any way to modify what this field looks like, at least not easily. Here are some links to places that might help
https://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/
https://stackoverflow.com/questions/572768/styling-an-input-type-file-button
