If you want to arrange the images in your gallery in a manual order, there are two ways to do this:
Go to your gallery settings, select “Sort by …” under “Arrangement,” and set it to “Custom” Then click on “Edit order.” You can now easily move your images using your mouse. Once you click “Save order,” the changes will take effect immediately.
Alternatively, open the gallery and use the left sidebar. Under “Arrangement,” choose “Sort by” and then “Custom” and click on “Edit order.” A dialog will open where you can arrange the images in your preferred order.
In both cases, the image order is then fixed for the gallery and will be visible to your clients as well.