I will answer to myself. I am also attaching the code if anyone ever needs it. If the website is a 'website' and not a DMS (if they are doing a DMS then they should zip them and deliver them as an link attachment) then it is not necessary to make the images larger than the maximum size for the screen. They are usually smaller! Our editors like to upload huge amounts of images, but they never use them. That's why I have a handler that, when uploading, reduces extremely large images to the appropriate size and at the same time converts them to webp format. And so this handler:

public class MediaSavingNotificationHandler : INotificationHandler<MediaSavingNotification>
{
    private readonly IMediaService _mediaService;
    private readonly MediaFileManager _mediaFileManager;

    public MediaSavingNotificationHandler(
        IMediaService mediaService,
        MediaFileManager mediaFileManager
    )
    {
        _mediaService = mediaService;
        _mediaFileManager = mediaFileManager;
    }

    public void Handle(MediaSavingNotification notification)
    {
        foreach (var mediaItem in notification.SavedEntities)
        {
            if (mediaItem.ContentType.Alias.Equals("Image"))
            {
                if (!mediaItem.HasIdentity)
                {
                    string filePath = string.Empty;

                    using (Stream stream = _mediaFileManager.GetFile(mediaItem, out string mediaFilePath))
                    {
                        filePath = _mediaFileManager.FileSystem.GetFullPath(mediaFilePath);
                    }

                    using (Image img = Image.Load(filePath))
                    {
                        // only if image is larger
                        if (img.Width > 2560 || img.Height > 2560)
                        {
                            img.Mutate(x =>
                                x.Resize(
                                    new ResizeOptions()
                                    {
                                        Mode = ResizeMode.Max,
                                        Size = new Size(2560, 2560)
                                    }
                                )
                            );
                            img.SaveAsWebp(filePath);

                            // get file info
                            FileInfo fi = new FileInfo(filePath);

                            // set new values
                            mediaItem.SetValue(Constants.Conventions.Media.Width, img.Width);
                            mediaItem.SetValue(Constants.Conventions.Media.Height, img.Height);
                            mediaItem.SetValue(Constants.Conventions.Media.Bytes, fi.Length);
                        }
                    };
                }
            }
        }
    }
}

First, I had a problem because I was additionally calling

mediaService.Save(mediaItem);

which is redundant here

Regards