One thing to try would be closing the image tag...
Code:
<img src='/path/to/image.gif' />
Another thing to try might be to do something with CSS...
Code:
<li class='spacer_line'><span></span></li>
then in your CSS:
Code:
li.spacer_line {
height:5px;
background-image:url("/path/to/image.gif");
background-repeat:no-repeat;
}