<!-- working image -->
<img src="https://picsum.photos/300/300" alt="">

<br/>

<!-- broken image -->
<img src="" alt="">

img {
  position: relative;
  width: 300px;
  height: 300px;
  margin-left:auto;
  margin-right:auto;
  object-fit: cover;
  display: block;
}

img:before {
  content: "The image could not be loaded.";
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  background: #000;
  text-align: center;
  font-family: Arial;
  font-size:18px;
  color: red;
}