To re-order images click the Add Media button.
Choose “Uploaded to this post” from the drop-down list. Make sure that you are looking at the Media Library screen and not the Upload Files screen.
The screen won’t look all that different, but only images in the post will appear.
Next, drag and drop the images to re-order them.
Close the image uploader by clicking the “X” in the upper right hand corner.
Be sure to click “Update” or “Publish” to save changes. Check the Post or Page to view the changed order.