Make sure the the containing element (wrapping div) has relative positioning applied.
div.mainRunner { position:relative;}
After this you can do one of the following.
Apply a class name to each image so you can map to it with absolute positioning.
div.mainRunner img.classname { position:absolute; top:__; left:__;}
Lastly apply z-index to the image class.
div.mainRunner img.classname { position:absolute; top:__; left:__; z-index:50;}
And for the second image;
div.mainRunner img.classname { position:absolute; top:__; left:__; z-index:51;}
If you have no control over applying classes to the images then do this (on the assumption that only 2 images will be in this div;
div.mainRunner img.classname:first-child { position:absolute; top:__; left:__; z-index:50;}
div.mainRunner img.classname { position:absolute; top:__; left:__; z-index:51;}
You would have to use positioning and z-index to get this to work, with changing the images to block level elements and adding a class:
.mainRunner {
border: 1px solid #000;
position: relative;
}
.img1 {
border: 1px solid #f00;
position: relative;
z-index: 2;
}
.img2 {
border: 1px solid #0f0;
position: relative;
z-index: 1;
top: -12px;
left: 12px;
}
<div class="mainRunner">
<img class="img1" src="http://t0.gstatic.com/images?q=tbn:ANd9GcTG4mTuuZmylqn_qqviXFh5EPLD_DTsXMIjXT-4XJM0QPtJxw7lXw&t=1" />
<img class="img2" src="http://t0.gstatic.com/images?q=tbn:ANd9GcTG4mTuuZmylqn_qqviXFh5EPLD_DTsXMIjXT-4XJM0QPtJxw7lXw&t=1" />
</div>
You could use absolute positioning (please don't), or negative margins with display:inline
.