I am sending base64 encoded image from client side using javascript (I am creating Screenshot uploader applet for asp.net application using http://supa.sourceforge.net/) and
First, you need to convert the base 64 back into bytes:
byte[] data = System.Convert.FromBase64String(fromBase64);
Then, you can load it into an instance of Image:
MemoryStream ms = new MemoryStream(data);
Image img = Image.FromStream(ms);
If you want to save it to a file instead, use System.IO.File.WriteAllBytes