How to resize image after being uploaded in ASP.NET Core 2.0

You could get nuget package SixLabors.ImageSharp (do not forget to tick "Include prereleases" since now they have only beta) and use they library like this. Their GitHub

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;

// Image.Load(string path) is a shortcut for our default type. 
// Other pixel formats use Image.Load<TPixel>(string path))
using (Image<Rgba32> image = Image.Load("foo.jpg"))
{
    image.Mutate(x => x
         .Resize(image.Width / 2, image.Height / 2)
         .Grayscale());
    image.Save("bar.jpg"); // Automatic encoder selected based on extension.
}

.NET Core 2.0 ships with System.Drawing.Common, which is the official implementation of System.Drawing for .NET Core.

Instead of CoreCompat.System.Drawing, can you try to install System.Drawing.Common and check whether that works?