img {
max-width: 100%;
height: auto;
}
<h2>dog1</h2>
<p><img src="https://placedog.net/400/300/1" alt="" width="400" height="300"></p>
<h2>dog2</h2>
<p><img src="https://placedog.net/400/300/2" alt="" width="400" height="300"></p>
<h2>dog3</h2>
<p><img src="https://placedog.net/400/300/3" alt="" width="400" height="300"></p>