Responsive Dimensions (%,em,rem,max-width) + FLEXBOX + MEDIAQUERY

The one thing where I am struggling nowadays is background image. I am not able to clone background images in most of the designs. When I start to code I look the design and in my mind I have a particular approach to proceed with except the backgroud image. I don't know most of the times how will I implement that. Any suggestion related to that will be helpful to me.