This code assumes that your input image is not larger then 1200x629. If you want to be able to handle larger images you should first resize it like this:
This method will resize the image to fit inside a rectangle that is 1200x629. And after the resize you should extent to image to 1200x629.
if (image.Width > 1200 || image.Height > 629)
image.Resize(1200, 629);