Bootstrap version 4 has issue with responsive images and that is the developers removed img-responsive
class from version 4. Why? ask them!
What I did is bringing back img-responsive
class to version 4. Now you can use this class for your images and be assure that your design will be 100% responsive
For test try to resize your browser size
Responsive Image
Non responsive Image
<img src="img-link" class="img responsive" >