I finally solved the problem. The cause of the problem is unprocessed
shortcodes in the html content, which is the {{ media url="..." }}
statement. Before echo this statement, the html content should be
process by the processor from helper "cms". Below is the code:
$html_content = $content_from_db['content'];
$html_content = Mage::helper('cms')->getPageTemplateProcessor()->filter($html_content);
echo $html_content;
