Check out example codes for "bootstrap Picture". It will help you in understanding the concepts better.

Code Example 1

In addition to our border-radius utilities, you can use 
.img-thumbnail to give an image a rounded 1px border appearance.

<img src="..." alt="..." class="img-thumbnail">

Code Example 2

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.

  <source srcset="..." type="image/svg+xml">
  <img src="..." class="img-fluid img-thumbnail" alt="...">

Code Example 3

<img src="..." alt="..." class="img-thumbnail">

Learn ReactJs, React Native from