Hi Mel99. I use Twitter Bootstrap also. Here are a couple possibilities I see...
1) For responsive pages I always start a new section like this:
Your example is missing the container-fluid class. That may not be affecting your well, but it sure is easy to test
2) It sounds like you want an image with a caption, all in a shaded box with round corners. You may need to use something besides a "well" to do that...
This would give you a captioned image...
...and I assume you know how to create the bordered box with a light background to put it into.
<img src="../images/some.jpg" />
<figcaption>Words for below the picture</figcaption>
3) Or you could try just forcing everything into position like this (not sure if it will work because I have never used their "wells")
Note that I moved the "well-text" class to include both the <h2> and the <p>. Also the "clearfix" may not be needed. Experiment and let us know the result.
<div class="span4 well" style="text-align:center;" >
<img src="img.png" />
<div class="well-text clearfix" style="text-align:left;" >
<h2><a href="#">Some text</a></h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, </p>
Maybe none of these work as-is, but they are food for thought