CCS code for background image Highend Theme

Hi I was having problems uploading a background image behind the text in my about page but after reading through other answers I can see that is only possible with box d layout…so now I am trying to use custom css code, but can’t seem to get the image how I want… I’m either getting tiled or not centred or stretched but distorted and what I’m really looking for is for a responsive image to fill the whole background and without getting distorted.
Could you please tell me what code I should be using for that?
Thank you
Angelio3