If you add that to the snippet call, then each link will have an ID of "itemx" where x is a unique number (I forget exactly if it’s the document ID or what, but it will be unique). Then in your CSS file, style the links so they have enough padding on the end for the background image to show, then assign each item its background image.
Another way you could do this is to make sure your images are named with the various document’s ID, or alias would also work. Then make up a rowTpl chunk:
<li><a href="[+wf.link+]" style="background:url(assets/images/[+wf.alias+].jpg) left center no-repeat;">[+wf.menutitle+]</a></li>
Or put the background to the <li> element, whatever you like. There’s probably at least four or five other ways you could work it out between the chunk templates and the CSS.