Server side image captions

Hi,
Sorry if this has been asked before – I wasn’t exactly sure what to search for so didn’t find much.

The easiest way to explain what I want to do is by using the example of smilies on wordpress. Enter ?? or ?? etc. and WordPress will convert it to an img tag on the server side.

What I want to do is use this feature to add a script that will convert something like this:
<img src="xyz.jpg" alt="A nice picture />
to something like this:

<div class="pic">
  <div class="pic-img">
    <img src="xyz.jpg" alt="A nice picture">
  </div>
  <div class="pic-capt">
    A nice picture
  </div>
</div>

Does anybody know how I could achieve this? I know it can be done on the client side with Javascript but I would prefer it done on the server side (at the same time as the smilies are done – see my example above).

Thanks a lot,
Rob