Background issues

Hi guys here is my site https://www.westcoastsurf.co.za and im trying to get the back ground to do this https://www.swellguys.co.za the image that i am using is exactly the same as i want to set it then change teh image to my own pls tell me how this is done im so confused i have tried many css routes and alas no victory PLS HELP!!

Thanks in advance