I’ve done a little digging on this, and it looks like this is because ImageSharp is built only for .NET Standard 1.1, 1.3, 2.0, and .NET Core 2.1.
According to this table, using .NET Standard requires mono 4.6 or later. Mac Rhino v5 is using mono 4.0.2.
Unfortunately, unless ImageSharp was built for .NET 4.5, it will probably not work with Mac Rhino v5.
Hope this helps!