Thanks for the suggestions kmessinger
I have now deactivated absolutely every plugin but it didnt solve the problem. I switched themes to Twenty Eleven and the extra thumbnail image was gone, so it's definitely the theme.
I deleted the theme and re-uploaded a fresh copy from the wordpress site. Still no good.
I used Firebug to identify the code where the duplicate image is and it's below, between the
<h2> and the
<p> I just dont know enough about wordpress themes to know where that is in the theme, or how it got in there, the double image wasnt there 2 weeks ago!
<div id="post-921" class="post-921 post type-post status-publish format-standard hentry category-after-effects-tutorials category-motion-graphics-tutorial tag-3d-camera-tracker tag-3d-projection tag-after-effects tag-motion-tracking tag-tracker">
<img class="alignleft wp-post-image" width="200" height="83" title="Projection-Camera-Tracker-Mapping-Tutorial.jpg" alt="Projection-Camera-Tracker-Mapping-Tutorial.jpg" src="http://www.my-website.com/blog/wp-content/uploads/2012/06/Projection-Camera-Tracker-Mapping-Tutorial-300x125.jpg">