Bootstrap 4 img class

Example

<img src="/upload/45/03143821507601.jpg" class="img-fluid">

Result:

Example

<img src="/upload/45/03143821507601_t.jpg" class="img-thumbnail">

Result:

Example

.img-fluid {
  max-width: 100%;
  height: auto;
}

.img-thumbnail {
  padding: 0.25rem;
  background-color: #fff;
  border: 1px solid #dee2e6;
  border-radius: 0.25rem;
  max-width: 100%;
  height: auto;
}