Disclaimer: This is my software.
I’m working on a cross-platform 2D Graphics library that runs on .NET Core It’s currently alpha but already supports a comprehensive feature set.
https://github.com/JimBobSquarePants/ImageSharp
Example usage.
using (FileStream stream = File.OpenRead("foo.jpg"))
using (FileStream output = File.OpenWrite("bar.jpg"))
{
Image image = new Image(stream);
image.Resize(image.Width / 2, image.Height / 2)
.Greyscale()
.Save(output);
}