Hi André

How about this?

using ImageProcessor;
using ImageProcessor.Imaging;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Web;
using Umbraco.Core;
using Umbraco.Core.Composing;
using Umbraco.Core.Configuration.UmbracoSettings;
using Umbraco.Core.IO;
using Umbraco.Core.Models;
using Umbraco.Core.PropertyEditors.ValueConverters;
using Umbraco.Core.Services;
using Umbraco.Core.Services.Implement;

namespace umbraco_8_1_release.Components
{
    [RuntimeLevel(MinLevel = RuntimeLevel.Run)]
    public class ImagePreprocessingComposer : ComponentComposer<ImagePreprocessingComponent>
    {
    }
    public class ImagePreprocessingComponent : IComponent
    {

        private readonly IMediaFileSystem _mediaFileSystem;
        private readonly IContentSection _contentSection;
        public ImagePreprocessingComponent(IMediaFileSystem mediaFileSystem, IContentSection contentSection)
        {
            _mediaFileSystem = mediaFileSystem;
            _contentSection = contentSection;
        }
        public void Initialize()
        {
            MediaService.Saving += MediaService_Saving;
        }

        public void Terminate()
        {

        }

        private void MediaService_Saving(IMediaService sender, Umbraco.Core.Events.SaveEventArgs<Umbraco.Core.Models.IMedia> e)
        {
            IEnumerable<string> supportedTypes = _contentSection.ImageFileTypes.ToList();
            foreach (IMedia media in e.SavedEntities)
            {
                if (media.HasProperty("umbracoFile"))
                {
                    //Make sure it's an image.
                    string cropInfo = media.GetValue<string>("umbracoFile");
                    string path = JsonConvert.DeserializeObject<ImageCropperValue>(cropInfo).Src;
                    string extension = Path.GetExtension(path).Substring(1);
                    if (supportedTypes.InvariantContains(extension))
                    {
                        //Resize the image to 1920px wide, height is driven by the aspect ratio of the image.
                        string fullPath = _mediaFileSystem.GetFullPath(path);
                        using (ImageFactory imageFactory = new ImageFactory(true))
                        {
                            ResizeLayer layer = new ResizeLayer(new Size(1920, 0), ResizeMode.Max)
                            {
                                Upscale = false
                            };

                            var image = imageFactory.Load(fullPath);
                            image.Resize(layer);
                            media.SetValue("umbracoWidth", image.Image.Width);
                            media.SetValue("umbracoHeight", image.Image.Height);
                            image.Save(fullPath);

                        }
                    }
                }
            }
        }
    }
}

Essentially create a new class in your project, and create a Component to setup the handling of the MediaService saving event, when Umbraco is initialized and then you can get access to the MediaFileSystem and ContentSection configuration elements by adding them as arguments to the public constructor for your component...

... and then the code is pretty much the same as you had before...

except if you are using ImageCropper as the UmbracoFile property will contain a blob of JSON instead of your file path...

.. and you'll want to adjust the new height and width to be displayed in the backoffice...

... so that's sort of how to do the same thing in V8 ...

Some documentation on composing here: https://our.umbraco.com/Documentation/Implementation/Composing/ and some examples of injection of services here: https://our.umbraco.com/Documentation/Implementation/Services/

might be of use.

regards

Marc