.gif instead of border-bottom

If you mean an image that sits between each post, then just call it like so:
<div class="feedback">
<?php link_pages('
Pages: ', '
', 'number'); ?>
<?php comments_popup_link('Comments (0)', 'Comments (1)', 'Comments (%)'); ?>
</div>
<img src="atoms.gif"/>
<!–
<?php trackback_rdf(); ?>
–>
That puts the image ‘atoms.gif’ below each post. You’d need to center it or whatever, but it works.

Remember that for validation purposes <img> would also need an ‘alt’ attribute. Tcervo’s answer is more elegant but may not work in all browsers. Personally as the image is purely decorative I would set it as a background to .feedback; what code were you trying for that?

or try
.feedback {
background: url(https://www.yoursite/images/border.gif) no-repeat 50% 50%;
}

Just a line of thought. A couple of guys here are indicating you have choices and what you want can be done several ways. As a matter of principle I always try not to edit the index or php primarily for reasons of upgrades. I have not quite achieved it but the index editing is down to two fixes. Secondly; odd as it may seem img may be depracated in css 3. It is very advanced thinking to call your img in the css as suggested by tcervo and it might well be the best way to go.

Just a line of thought. A couple of guys here are indicating you have choices and what you want can be done several ways. As a matter of principle I always try not to edit the index or php primarily for reasons of upgrades. I have not quite achieved it but the index editing is down to two fixes. Secondly; odd as it may seem img may be deprecated in css 3. It is very advanced thinking to call your img in the css as suggested by tcervo and it might well be the best way to go.

Thanks guys! I’m at work at the moment and can’t wait to come home and try this out!

WillM,
I’ve tested this type of solution in all the major (and most minor) browsers…works fine. It doesn’t work in v4 browsers, but then most of us are not serving them style sheets anyway. The nice thing about any browser that doesn’t get the style, is they will see a regular HTML horizontal rule instead (which serves the purpose of breaking up the individual posts).
And I do agree with anon that trying to limit changes to the index.php file is a good thing. Sometimes, though, it just makes sense to do so. A good idea might be to preface any addition or change with a comment <– added by me –> so that when a new major upgrade comes along, if your old index.php isn’t working, you can easily find and add back the little changes (like adding an hr tag) to the new and improved index.php.
-Tony