Images
You can use normal images or responsive images.
Responsive images
Images are made responsive with .img-fluid (max-width: 100%) and
height: auto. With this setup the image scales with the parent element.
<img src="..." class="img-fluid" alt="Responsive image">
Image thumbnails
You can use .img-thumbnail to give an image a rounded 1px border appearance.
<img src="..." class="img-thumbnail" alt="Thumbnail">
You can use Bootstrap utilities to make an image or thumbnail rounded:
<img src="..." class="img-thumbnail rounded-circle" alt="Thumbnail">
View the other possibilities of use for images in the official documentation.