Hi,
The code Xavier gave you displays the full path of the image.
That's what you asked in your original message and thus that's what you get.
However, reading your first message, what you want is not the code to get the full path of the image but the code to get the HTML for the display of the image.
In that case, it's that code that you want:
<?php
$db = JFactory::getDBO();
$db->setQuery('SELECT * FROM '.hikashop_table('file').' WHERE file_type = "category" AND file_ref_id = '.(int)$this->element->product_manufacturer_id);
$image = $db->loadObject();
$fileClass = hikashop_get('helper.image');
$img = $fileClass->getThumbnail($image->file_path, array('width' => '100', 'height' => '100'), array('default' => true,'forcesize'=>$this->config->get('image_force_size',true),'scale'=>$this->config->get('image_scale_mode','inside')));
echo '<img src="'.$img->url.'/>';
?>