I am trying to reference System.Drawing in a .net core console app targeting net46 but the assembly is not there. According to MS if you use dotnetcore System.Drawing is not
System.Drawing is not a good option with ASP.NET (more here: http://www.asprangers.com/post/2012/03/23/Why-you-should-not-use-SystemDrawing-from-ASPNET-applications.aspx)
If you want graphics manipulation i suggest to use ImageSharp (https://github.com/JimBobSquarePants/ImageSharp) on .Net Core or ImageProcessor (http://imageprocessor.org/) / ImageResizer (https://imageresizing.net/) on .Net Framework
Also, if you really need to use System.Drawing change frameworks in your project.json to netstandard1.6 and add in dependencies "NETStandard.Library": "1.6.1"