Saving images to website images folder

I've got some code that produces graphs that website users can then scroll through. However, I can't get them to save to the website images folder unless I put in the full address.
ie.
bitmapGraph.Save("E:\Interface\Images\Day.gif", ImageFormat.Gif)

Which is obviously fully dependant on always being run from the same place, which it won't be. I can get it to read from the image folder using

ImageUrl="~/Images/Day.gif"

This hasn't worked when I've tried using it for saving. Is there some other way of coding this to achieve the same sort of locational indep ...