(1) Clearly, you forgot to include the graphics library in your code. Bitmap isn't defined anywhere, so of course it can't be used.
(2) ".cs"??? What does that have to do with anything? ".cs" would be the extension for C# *source code*. You are coding in VB.NET, not C#. And in any case, the correct extension for any ASP.NET page is ".aspx". Yes, you can have ".cs" and ".vb" files in your project, but you can't serve those up to a web server.
Are you not using Visual Studio to build your code? At least Visual Web Developer Express (the free version)? IF not, you need to.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.