Here is your solution:
Issue:- Height value in percentage doesn't work, until unless it has been used in any position layer.
Solution:- Instead of height. Use "padding-bottom" OR "padding-top" value to make height responsive. Becasue percentage values work well with "padding".
The calculation is very simple to get the relevant responsive height value of any background image.
For Example:- If Background image dimensions are (1200 width) x (450 height) Pixels. Then image responsive "height" value would be: "37.5%"
Formula:- Y (×) 100 (÷) X. Which is (450×100÷1200) = 37.5%
Solution URL:- http://sandeepparashar.com/stackoverflow/responsive-css-background-image-height.html
Code:-
.custom-header-option {
padding-bottom:37.5%; /* This will make height responsive */
background-image: url('http://unplugged.ee/wp-content/uploads/2013/03/frank2.jpg');
background-repeat:no-repeat;
background-size:cover;
background-position:center;
}
If need more help, most welcome :)