Responsive theme image resize problem

Hi, I’m looking for some advise on an image issue when my site is viewed in mobile form. The images in question are flag icons which I would like to stay the same size, or at least a lot smaller than they look when viewed in mobile form.
The issue I’m having is the flag icons resize to the width of the screen, and as a consequence look really blurry and horrible.
Is there a way to stop these icons from resizing?
You can see the problem here (but only when viewed on a mobile device) – https://activitytrackerwear.com/fitbit-flex-review-worth-fuss/

Any help would be greatly appreciated.