goto: <a href="http://your_site.com/admin/content/node-type/your_content_type/display">your_site.com/admin/content/node-type/your_content_type/display</a><br><br>on the right side you will see under Full Node you image field. select the image type you want it to display, and click Save.<br>
<br>This should work if you are using the Content Construction Kit (CCK) for you content type.<br>