D7: Comments: Get access via foo_preprocess_comment(&$variables) to the comment authors profile image from an ($user) imagefield and print it into the comment.tpl.php template.

comment.tpl.php

<? if (!empty($image)) : ?>
    <img src="<?= image_style_url('mini',$image); ?>" alt="Profile image" />
<?php endif; ?>

template.php

/**
 * Preprocess comment
 */
function foo_preprocess_comment(&$variables) 
{
    $comment = $variables['comment'];
    $userID = $comment->uid;
    if ($userID != '0') {
        $user = user_load($userID);
        $profileImage = field_get_items('user', $user, 'field_user_image');
        if ($profileImage) {
            $variables['image'] = $profileImage[0]['uri'];
        } else {
            $variables['image'] = '';
        }
    }
}