If you are using .NET framework then it should be straight forward:
method-id Base64ToJpg (targetJpgPath as string, base64string as string).
declare imageBytes = type Convert::FromBase64String(base64string)
invoke type File::WriteAllBytes(targetJpgPath, imageBytes)
Assuming your base64 string is representing a JPEG blob. If it is another image format such as .png or .bmp then you will need to convert it first (you can do this using the Bitmap class in System.Drawing).