If you are using VB6 then you should not be able to use the .net framework, both VB.net and C# can though. System.Drawing.Dll is part of the .net framework and I believe that GDI+ is part of the Com framework and I know that you can only use GDI+ through C/C++: http://msdn.microsoft.com/en-us/library/ms533798(VS.85).aspx

I recommend learning DirectX8, it works nicely with VB6 and its fun and powerful. Despite being scary at first, I heartly recommend it!