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