Easiest way excpecially if you don't know images widths is to put the caption in it's own div element an define it to be cleared:both !
...
<div class="pics">
<img class="marq" src="pic_1.jpg" />
<div class="caption">My image 1</div>
</div>
<div class="pics">
<img class="marq" src="pic_2.jpg" />
<div class="caption">My image 2</div>
</div>
...
and in style-block define
div.caption: {
float: left;
clear: both;
}