Put html in variable_get and Email
What you basically need to do is add headers to your email. You can implement a solution like the one that can be found in this Drupal.org's comment. The code is pretty much straightforward:

$body = '&lt;html&gt;&lt;body&gt;&lt;img src="http://sitename.com/sites/all/images/logo.gif" /&gt;';
$body .= variable_get('email_content');
$body .= '&lt;/body&gt;&lt;/html&gt;';
$my_module = 'foo';
$my_mail_token = 'bar';  // A key to identify the e-mail sent
$from = 'tanvir@gmail.com';
$message = array(
  'to' =&gt; '"'.addslashes(mime_header_encode('Winnie Pooh')) .'"&lt;james@gmail.com&gt;',
  'subject' =&gt; t('Example subject'),
  'body' =&gt; $body,
  'headers' =&gt; array(
    'From' =&gt; $from,
    'MIME-Version' =&gt; '1.0',
    'Content-Type' =&gt; 'text/html;charset=utf-8',),
  );
$system = drupal_mail_system($my_module, $my_mail_token);
  if ($system-&gt;mail($message)) {
    // Success.
  } else {
    // Failure.
  }


Keep in mind that if your needs for html mailing become more complex and you need to theme your mails using template files or filter the mails' contents after the theme has applied, you can check the HTML Mail module.