You have mixed single and double quotes inside the single-quoted content string. The single quotes in the content are terminating the string and causing a PHP error, which would be in the error log if you have one. What version of Zen Cart do you have? Older versions do not have debug logging built in. If this is the case for you, you urgently need to upgrade to a newer version or else you will get hacked by the first hacker who finds your site. The older versions (v1.3.8a and older) have a serious security vulnerability that was discovered (and patched) in June 2009.
For your content, either replace the single quotes with double quotes, or escape them like this: \' .
PHP Code:
$content .= '
<div>
<div id='sealImage' imageSize='110' sealType='1'></div>
<script language="javascript" src='//seal.
PHP Code:
$content .= '
<div>
<div id="sealImage" imageSize="110" sealType="1"></div>
<script language="javascript" src="//seal.(etc...)
PHP Code:
$content .= '
<div>
<div id=\'sealImage\' imageSize=\'110\' sealType=\'1\'></div>
<script language="javascript" src=\'//seal.(etc...)