Get the centorid of the image by using mean. So I assume you know all the points A,B,C,D and O. You have (x,y) in hand. If you want angle w.r.t x-axes..simply find slope and get tan inverse of it.
th = atan((y2-y1)/(x2-x1)) ;
If you want angle between two lines...say OA and OB, ude dot product.
cos(th) = oa.ob/(|oa|*|ob|) ;