<img src="https://dummyimage.com/600x400/000/fff" alt="placeholder image" class="img-fluid" />
In Bootstrap 4 you would use the
.img-fluid class on all images you would like to be responsive. This class tells the browser not to expand the image larger than its original size using a max-width. And it also tells it to scale down the image if the browser window gets narrower than the image pixel width.
Read the Bootstrap 4 documentation for more information.