Minimal Menu Background modification

I'm having some problems on making my header image responsive to diferent screens since is not fixed or 100% width. I have tried adding it to the ccs code in many ways but im not geting any solution.


This is what im doing.

Muffin Options>Header&Subheader>Header> Classic Menu ( css code .header-classic)

This is the code I use for placing a background on my desktop web.
".header-classic #Top_bar { background: url(/2017/02/BG4.png);}"

I want to do the same on my footer and on my responsive menu.

The responsive menu I use is
Muffin Options>Responsive>Header>Layouts> Menu right l Left Logo

What is the css code I should use, and how do I do the background image responsive on different mobile dimensions.