How to apply an image style to an image field within a node template
I found this code somewhere as well and it seems to work within the template file.

 print theme('image_style', array( 'path' =&gt;  $field_my_image[0]['uri'], 'style_name' =&gt; 'square_550'));


where the field_my_image is the name of your field  ....  I suppose it does the same as the above code.