The simple resolution to this common problem is to click save on either the top right or bottom right of the screen. Once this has saved you should be able to use the blue arrows on the side of the images to re-order them. Move them by holding down the left mouse button over the arrow and moving your mouse up or down.
This applies to those who input their properties directly into the Homeflow CMS. If you use any estate agency software to upload your properties, it is advised you make the changes there.