If your web browser's zoom level is not set to 90 or 100%, it can lead to problems when you try to change the order of photos in Manage Photos. This generally occurs in Google Chrome.
To check your zoom level in Chrome, click the three dots in the upper right corner of the browser window. A menu will open and the Zoom level will display about 7 lines down in the menu:
Use the +/- to set it to 90 or 100%.
You can also hold CTRL on your keyboard and hit the + or - keys to change the zoom (oh hold CTRL and scroll forward or backward on your mouse).