This is a general issue with ASP.Net and not specific to DotNetNuke.  In order to use Server.MapPath, you have to be in a page or control class which exposes the Server object.  The server is just a property of HttpContext so if you have the context then you will have access to the Server object.  The solution is to use HttpContext.Current which returns an HttpContext object.

hmm can i use resolveurl something like...

 Image myImage = new Image();
myImage.ImageUrl =DotNetNuke.Common.ResolveUrl("desktopmodules....");

Bitmap b = new Bitmap(myImage);