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;
}