Check this one out:
https://www.markeverard.com/2014/02/13/image-resizing-in-episerver-7-5-cms/

Sven Tegelmo, the biggest issue on our site is not image size, it's quality. Our editors upload images which sometimes can be reduced by 70% without visible quality loss...

You can add on your own funktionality to handle size, quality, scale, ratio etc.

For instance do we use it like this in child class of ImageData:

[ScaffoldColumn(false)]
[ImageWidthDescriptor(Width = 1200)]
[ImageQualityDescriptor(JpegQuality = 85)]
public virtual Blob Large { get; set; }

By adding this do we keep the original upload image (good to have to be enable to reapply image optimization) as well as out optimized one (we actually have several with different size and quality depending on where its used).

Then hook on to the PublishingContent event on IConenteEvents and you can change the image in what ever way you like (in our case based on the atteributes set).

Sven Tegelmo Is it possible for you to show more detailed code? I'm not episerver ninja yet :D

Heres the handler for the event.
Not the prettiest code due to updates through episerver updates. And currenlty it's using an ugky swap of imageservice :-S The code has however worked fine for a long while so it's been untouched.
It need some refactoring and it actually uses an internal classes from Episerver for the image and blob generation. I think I'll replace them with better ones if they change... and do some refactoring.

public void contentEvents_PublishingContent(object sender, ContentEventArgs e)
{
  var content = e.Content;
  if (!(content is ImageData))
    return;

  var image = content as ImageData;
  Dimensions imageDimensions = ImageBlobUtility.GetDimensions(image.BinaryData);

  PropertyInfo[] properties = image.GetType().GetProperties();
  int requiredWidth;
  int requiredJpegQuality;

  foreach (var propertyInfo in properties)
  {
     if (propertyInfo.PropertyType != typeof(Blob))
       continue;

     // Returnera direkt om episerver original blob eller tumme
     if (propertyInfo.Name.Equals("Thumbnail") || propertyInfo.Name.Equals("BinaryData"))
       continue;

     // Defaults
     requiredWidth = imageDimensions.Width;
     requiredJpegQuality = 90; // Default

     //get attribute name
     var imageWidthAttribute = (ImageWidthDescriptorAttribute)Attribute.GetCustomAttribute(propertyInfo, typeof(ImageWidthDescriptorAttribute));
     var imageScaleAttribute = (ImageScaleDescriptorAttribute)Attribute.GetCustomAttribute(propertyInfo, typeof(ImageScaleDescriptorAttribute));
     var imageQualityAttribute = (ImageQualityDescriptorAttribute)Attribute.GetCustomAttribute(propertyInfo, typeof(ImageQualityDescriptorAttribute));

     if (imageWidthAttribute != null)
    {
       requiredWidth = imageWidthAttribute.Width;
    }
    else if (imageScaleAttribute != null)
    {
       requiredWidth = (int)(imageDimensions.Width * imageScaleAttribute.DimensionMultiplier);
    }
    if (imageQualityAttribute != null)
    {
       requiredJpegQuality = imageQualityAttribute.JpegQuality;
    }

    var calculatedDimensions = ImageResizeUtility.ResizeWidthMaintainAspectRatio(imageDimensions, requiredWidth);
    var imageDescriptor = new ImageDescriptorAttribute(calculatedDimensions.Height, calculatedDimensions.Width);

    var thumbnailManager = ServiceLocator.Current.GetInstance<ThumbnailManager>();
    var orgImageService = thumbnailManager.ImageService;

    try
    {
       // Använd min egen ImageService (Passa dock in original servicen dit då vi använder Epis under ytan)
       thumbnailManager.ImageService = new SFImageService(orgImageService, 1.0f, (int)requiredJpegQuality);

       // Gör om bild
      Blob resizedBlob = thumbnailManager.CreateImageBlob(image.BinaryData, propertyInfo.Name, imageDescriptor);

      // Använder ovanstående from Epi 10
      //Blob resizedBlob = CreateImageBlob(thumbnailManager, image.BinaryData, propertyInfo.Name, imageDescriptor, requiredJpegQuality);

      // Stoppa in blob på egenskapen
      propertyInfo.SetValue(image, resizedBlob, null);
    }
    finally
   {
      // Sätt tillbaka originalservicen. Vid fel eller inte.
      thumbnailManager.ImageService = orgImageService;
   }
 }
}

The SFImageService:

public class SFImageService : IImageService
{
IImageService _imageService;
float _zoomFactor;
int _jpegQuality;
public SFImageService(IImageService imageService, float zoomFactor, int jpegQuality)
{
_imageService = imageService;
_zoomFactor = zoomFactor;
_jpegQuality = jpegQuality;
}

public byte[] RenderImage(byte[] imageBuffer, IEnumerable<ImageOperation> operations, string mimeType, float zoomFactor, int jpegQuality)
{
   // Använd EpiServers egen...
   return _imageService.RenderImage(imageBuffer, operations, "image/jpeg", _zoomFactor, _jpegQuality);
}

great, thanks ;)

No problem. As i said it uses Internal Episerver classes so it might break. So you should really use another.... and I as well. I have also seen that the episerver one is not so good when it comes to image quality. If you stumble upon a good free one please let me know and I might swap as well :)

By the way: As described earlier do I add a new property to my image class. And therefore is the original image stored as well as the modified/optimized one. (Actually do I have three more in different sizes to be efficient on mobiles etc). A good thing is that if you change how the image is generated/optimzed can they be regenerated (or actually cleared) easily i the Episerver admin GUI. The "Clear Thumbnail Properties" schedule job does that for you.

Hi,

Maybe an option is to use my Episerver ImageProcessor add-on (blog, package) in combination with the ImageProcessor Post Processor plugin (http://imageprocessor.org/imageprocessor-web/plugins/postprocessor/)?

Hi, in the meantime I found other solution which is most suitable for us. 
https://www.david-tec.com/2017/02/using-tinypng-to-automatically-compress-images-in-episerver/

https://tinypng.com/

Works great, there's completely no difference between original and compressed images from visual perspective. And it's really cheap. 

Thanks everyone for help!