Responsive images
Images in Bootstrap are made responsive with .img-fluid. This applies max-width: 100%; and height: auto; to the image so that it scales with the parent width.
Image thumbnails
In addition to our border-radius utilities, you can use .img-thumbnail to give an image a rounded 1px border appearance.
Aligning images
Align images with the helper float classes or text alignment classes. block-level images can be centered using the .mx-auto margin utility class.
Picture
If you are using the <picture> element to specify multiple <source> elements for a specific <img>, make sure to add the .img-* classes to the <img> and not to the <picture> tag.
<picture>
<source srcset="..." type="image/svg+xml">
<img src="..." class="img-fluid img-thumbnail" alt="...">
</picture>
Sass
Variables
Variables are available for image thumbnails.
$thumbnail-padding: .25rem;
$thumbnail-bg: var(--#{$prefix}body-bg);
$thumbnail-border-width: var(--#{$prefix}border-width);
$thumbnail-border-color: var(--#{$prefix}border-color);
$thumbnail-border-radius: var(--#{$prefix}border-radius);
$thumbnail-box-shadow: var(--#{$prefix}box-shadow-sm);