show image field twice with different image style
If you're displaying your field programmatically you've to use: 
For Large :

&lt;?php echo theme('image_style', array(
    'style_name' =&gt; 'large',
    'path' =&gt; $node-&gt;field_image['und'][0]['uri'],
    'alt' =&gt; t('Image'),
    'title' =&gt; t('Title'))); ?&gt;


For Medium : 

&lt;?php echo theme('image_style', array(
    'style_name' =&gt; 'medium',
    'path' =&gt; $node-&gt;field_image['und'][0]['uri'],
    'alt' =&gt; t('Image'),
    'title' =&gt; t('Title'))); ?&gt;


Your can also call any format you added on back office his name