Problems with .NET Framework references in Unity Script

I created a simple script (a heightmap generator) which uses the reference System.Drawing and tried to compile it with Unity, but all the time when I started Unity my reference was gone and Unity started with an error: “missing Assembly reference”. I loaded the same project again a few times with Visual C# (Express) and re-added the reference (.NET → System.Drawing). But then Unity loaded the project again with the same error. Can anyone help?

adding it on the sln side is of no use. untiy does not use it at all, its only there for your to use monodevelop / VS

if you want to add it as reference, add the corresponding assembly from your unity folder to the project

thanks for your response.
you mean i should copy the .sln file from the main project folder to the assets folder (?), is that right, or which “Unity folder” do you mean? I would be very pleased if you could specify your answer a bit (I’m quite new to Unity you know).
(Sorry if my english insn’t that good)