thumbnail image size bootstrap code example
Example 1: bootstrap 4 center image
/* used mx-auto d-block */
<img class="mx-auto d-block" src="https://www.w3schools.com/bootstrap4/paris.jpg" alt="https://www.w3schools.com/bootstrap4/paris.jpg"/>
Example 2: bootstrap image size
.food1 img {
width:100%;
height: 230px;
}