Well you only want the box to expand vertically so you need the code to have the right number of elements to apply the css to.
You need the top two corners then the middle to repeat and then the bottom two corners as images. So thats images to apply to three elements.
<div id=topCorners">
<div id="botCorners">
<p>Content here to expand<p>
</div>
</div>
CSS should be something like:
#topCorners { background: url(images/topcorners.gif) top no-repeat; }
#topCorners p { background: url(images/leftandrightsides.gif) repeat-y; }
#botCorners { background: url(images/bottomcorners.gif) bottom no-repeat; }
The middle image can be 1px high and will repeat with the content of the paragraph.
You'll find that this most likely wont all align correctly so you may have to play with the margin and padding a bit. Set it to 0 on all three elements and then see how it behaves before you adjust them.