thumbnail class bootstrap code example

Example 1: image responsive bootstrap 4

<img class="img-fluid" src="" alt=""> //This will make your image responsive

Example 2: 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">

Example 3: bootstrap image size

.food1 img {
    width:100%;
    height: 230px;
}