Actually using 100% will not make the image bigger if the image is smaller than the div size you specified. You need to set one of the dimensions, height or width in order to have all images fill the space. In my experience it's better to have the height set so each row is the same size, then all items wrap to next line properly. This will produce an output similar to fotolia.com (stock image website)
with css:
parent {
width: 42px; /* I took the width from your post and placed it in css */
height: 42px;
}
/* This will style any <img> element in .parent div */
.parent img {
height: 42px;
}
without:
<div style="height:42px;width:42px">
<img style="height:42px" src="http://someimage.jpg">
</div>