This was declined on the basis that a single image can exist in multiple galleries so implementing a weighting system will mean that the weight of an image will remain the same regardless of which gallery is in.
if this was to reach in the future the threshold of 300 people as well it will be considered to be implemented as a slightly wonky weighting system. Keep in mind that as it is, images are ordered alphanumerically. So there are ways to order them as it is.As per a previous request (HBFR-6176), I agree that we should be able to organize galleries to our liking.
Why aren't images sorted by filename already? The server's operating system already does this because all of them do. All that's needed is to display images in the order they are already listed by the file system by doing nothing to sort them, thereby reducing computational overhead.
I can see how this would be useful to some people, but I'm not sure how many people would actually get any real value from it. That said, my guess is that it would only require some sort of "weight" value added to each image, then you could choose to arrange by weight, name, or date added, and that could be really useful.