Do something like this:
Code:
<p>
By clicking the slug at left, a you can hide and show a little box that displays the slug you can use in posts.
<span class="slug" style="position: relative;"><img src="slugs_files/images/slug.gif" alt="(SLUG)" title="Show/Hide slug" /><span class="show" style="position: absolute; top: 0; left: 10px;">slug:this/should/hover/over/the/text@right</span></span>
The slug address, when shown, should cover up this text (or at least the start of it).
</p>
The styles are inline. Basically, you just give the box an absolute positioning X (didn't check how many) pixels left. Because of a CSS quirck/firefox bug, you have to specify explicitly that the containing span (class="slug") is positioned relatively.