How to change an image field to a certain fid programatically?
You can do this in the comment tpl file. Add something like this:

$img_array = array(
    'style_name' =&gt; 'thumbnail',
    'path' =&gt; $user-&gt;field_picture['und'][0]['uri'],
    'width' =&gt; '',
    'height' =&gt; '',
    'alt' =&gt; $user-&gt;field_picture['und'][0]['alt'],
    'title' =&gt; $user-&gt;field_picture['und'][0]['title'],
  );

$img = theme('image_style', $img_array);


You can then add &lt;?php print $img; ?&gt; where you want the image to appear.