CSS

one image position relative and other absolute

<div>
    <img src="http://placekitten.com/300/300">    
    <img src="http://placekitten.com/30/30" id="smallone">
</div>

/*CSS*/
div{
    float: left;
    position: relative;
}

img{
     vertical-align: bottom;   
}

#smallone{
    top: 0;
    left:0;
    position:absolute;   
}