Check out example codes for "html css how to arrange images of different sizes". It will help you in understanding the concepts better.

Code Example 1

.flex-container {
    flex-flow: row;
    justify-content: space-between;
    align-items: flex-start; /* should align items to the top of your flex row if they don't reach 100px in height */
}
.inner-image {
    max-height: 100px;
}
.inner-image.portrait {
    height: 200px;
    width: 100px;
}
.inner-image.landscape {
    height: 100px;
    width: 200px;
}

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