Answers for "how to make a image responsive in bootstrap"

responsive image in bootstrap

<img src="..." class="img-responsive">

bootstrap img thumbnail

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

bootstrap img src

<img src="..." class="rounded mx-auto d-block" alt="...">

bootstrap Image thumbnails

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">