How to set the image size dynamically fit the browser size?

I not sure is my description above correct. But this few image below hope can elaborate what I mean.

On the new premiumpress website design, the city vector show on the main page, when I open my browser wide enough or restore down my browser to be smaller, the city vector follow the browser size.

While in my business directory theme, I also place a city vector on the main page, but when I restore down my browser size to smaller, the city vector does not follow the browser size.

any one can suggest me how can I make the city vector follow the browser size when I maximize/restore down to difference size?