Put html in variable_get and Email I am using custom form to store email content using variable_set.
Using drupal_mail and hook_mail to send that content using variable_get.
Now in email I want to add image using html.

$pre_body = '&lt;img src="http://sitename.com/sites/all/images/logo.gif" /&gt;&lt;div style="padding-top: 10px;"&gt;'
$body = variable_get('email_content');
$post_body = '&lt;/div&gt;';
$full_body = $pre_body . $body . $post_body;
$message['body'][] = t('@body', array('@body' =&gt; $full_body));


but in email it shows me...

&lt;img src="http://sitename.com/sites/all/images/logo.gif" /&gt;
&lt;div style="padding-top: 10px;"&gt;Hello obv
You have placed a bid on lot 'AA' of auction 'Test Curator Email'.
You currently the highest bidder.&lt;/div&gt; 


how to achieve it...???
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.