You have a few options.  All of them include some amount of coding.  There isn't a built in way for SQL Server to export base64 encoded strings as files.  
However you can....
Write a CLR that will do it from the TSQL / database side.  The relevant C# code you would need in the CLR is something like this:
File.WriteAllBytes(@"c:\yourfile", Convert.FromBase64String(yourBase64String));
This is courtesy of this link: Base64 encoded string to file
Or write a quick console application to run through each of your rows and pull it out.  Using the same code snippet above for C# or any other number of languages that are documented out on google.  Another method a quick google search found is:
public Image Base64ToImage(string base64String)
{
  // Convert Base64 String to byte[]
  byte[] imageBytes = Convert.FromBase64String(base64String);
  MemoryStream ms = new MemoryStream(imageBytes, 0, 
    imageBytes.Length);
  // Convert byte[] to Image
  ms.Write(imageBytes, 0, imageBytes.Length);
  Image image = Image.FromStream(ms, true);
  return image;
}
Courtesy of here: http://www.dailycoding.com/Posts/convert_image_to_base64_string_and_base64_string_to_image.aspx