David, you had the right idea to start with. To delete an image file, you have to detach all the pages that are attached to it. That will immediately remove it from any pages on which it had been shown, but the image file will still be here for a few days, in case you want to re-claim it. After that, it will be permanently deleted from the server.
If you don't want to wait to upload a new version of the file, you can change its name in your computer - that way WikiTree won't tell you it already exists. It doesn't matter at all what your name is for the file - as you probably noticed, WikiTree will give it a name to match the page you first upload it to, followed by a hyphen and a number, then the extension (like .jpg, .png, or .gif).
WikiTree does not have any graphic editing capability so the only way to rotate an image is to do it in your computer and save it, then upload the new version.