Edit in JSFiddle

<div class="flexTest">
    <img src="holder.js/300x200/auto/random"></img>
</div>

<div class="flexTest">
    <img src="holder.js/200x300/auto/random"></img>
</div>

<div class="flexTest">
    <img src="holder.js/200x200/auto/random"></img>
</div>

<div class="flexTest">
    <img src="holder.js/100x100/auto/random"></img>
</div>

*{box-sizing:border-box;}
.flexTest{
    display:flex;
    width:200px;
    height:200px;
    padding:5px;
    float: left;
    margin-left:20px;
    border:1px solid #ddd;
    justify-content:center;
    align-items:center;
}
.flexTest img{
    max-width:100%;
    max-height:100%;
    width:auto;
    height:auto;
}