You can use a .change event, and smart file naming to achieve this result. Using jQuery is by far the easiest solution.
If you name your images the same as the option values, you can then reference images based off of the current option selected. So for the first option you could name the picture, p1.jpg.
<label for="pictype">Select a New Background</label>
<select id="pictype" name="pictype">
<option value="p1">pic1</option>
<option value="p2">pic2</option>
<option value="p3">pic3</option>
<option value="p4">pic4</option>
<option value="p5">pic5</option>
<option value="p6">pic6</option>
</select>
jQuery
$("#pictype").change(function(){
var imageFileName = $(this).val();
$("body").css("background-image", "url(../Images/"+imageFileName+".jpg)");
});
So if you selected option #1, it would set the background image url to url("../Images/p1.jpg");
Here is the jsfiddle: http://jsfiddle.net/mtruty/3L2uP/
Hope this helps!