Grab first  or  src from Rich Text field
I had the exact same issue with a client and their horrendous Wordpress blog posts.

In the end, after importing all the posts and copying over the related images, I wrote a simple plugin that lets you use a |firstImage filter to grab the first image from a rich text field. I've added it to GitHub: https://github.com/DigitalSquid/Craft-CMS-First-Image

A rough example of how to use it:
{% set firstImage = entry.body|firstImage %}
{% if firstImage|length %}
    &lt;img src="{{ firstImage }}" /&gt;
{% endif %}