Creating a responsive image in Bootstrap 4 is a breeze:
All you have to do is include the img-fluid class with the image.
However, there's a tradeoff for responsiveness – it means you can't specify a minimum height for a responsive image (as that would defeat the purpose of being responsive).
Fortunately, there are some ways to control the size of your card image while still keeping it responsive. You can achieve this by adjusting the horizontal padding (using the px-* class and substituting the * with a number between 0 and 5) as well as altering the column width.
Check out these 3 examples below:
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<div class="container">
<div class="row mt-3">
<div class="col-md-6">
<div class="card">
<img class="card-img-top img-fluid" src="https://placeimg.com/640/480/animals" alt="Card image cap">
<div class="card-block text-center">
<p class="card-text">Text goes here</p>
<p class="card-subtitle">More text goes here</p>
</div>
</div>
</div>
</div>
<div class="row mt-3">
<div class="col-md-6 px-5">
<div class="card">
<img class="card-img-top img-fluid" src="https://placeimg.com/640/480/animals" alt="Card image cap">
<div class="card-block text-center">
<p class="card-text">Column with px-5 class</p>
<p class="card-subtitle">to make the image smaller</p>
</div>
</div>
</div>
</div>
<div class="row mt-3">
<div class="col-5 px-5">
<div class="card">
<img class="card-img-top img-fluid" src="https://placeimg.com/640/480/animals" alt="Card image cap">
<div class="card-block text-center">
<p class="card-text">Smaller column AND px-5 class</p>
<p class="card-subtitle">to make the image even smaller!</p>
</div>
</div>
</div>
</div>
</div>