Change post body

I had imported my posts from blogger. Body of almost all this posts start like this:

<div style="...">
  <a href="..."><img src=".." /></a>
</div>

Bla, bla, bla
...

and I need to add <br clear="left" /> after the </div> tag.

So:

• loop through all posts
• chech if body starts with an image
• insert code after the image

Someone can help me?

Thank you very much!