Responsive stats card component

I have tried to make the page as responsive as possible by writing media query for 3 screen sizes. My questions

1. which is better for the image on the left, putting an empty div on the html and filling it with image using url(), which I did because it is easier to switch to another image for mobile view, or actually putting img tag on html, which I don't see any methods to switch to another img when the screen size is smaller.
2. does the view change to mobile when the screen is below 375px or does it end there? I couldn't understand where to start and end my media query from the style guide file.