Image_style_url with [default_image]
Use below code in else condition...

$user = user_load($comment-&gt;uid);
print theme('user_picture', array('account' =&gt; $user));


OR

$filepath = variable_get('user_picture_default', '');
$default_image = theme('image_style', array('style_name' =&gt; $style, 'path' =&gt; $filepath, 'alt' =&gt; '', 'title' =&gt; ''));
print $default_image;