[kh99]

You can use a php type block and make the html into strings (and the condition would then be a php 'if'), like:

$output = '<center><a href="http://www.midwiferyonline.co.uk/store/index.php/midwife-in-the-making-hoodie-7/hoodies.html">
<img src="http://www.midwiferyonline.co.uk/images/ads/members10discount.jpg" alt="10% OFF Hoodies" width="180" height="220"></a>
<br /><br />';
global $show;
if ($show['guest'])
{
    $output .= '<a href="http://www.midwiferyonline.co.uk/register.php">Register</a> or <a href="http://www.midwiferyonline.co.uk/login.php?do=login">Login</a>';
}
else
{
    $output .= '<img src="http://www.midwiferyonline.co.uk/images/ads/members10discountcoupon.jpg" alt="10% OFF Hoodies" width="155" height="166"></a>';
}
$output .= '</center>';

return $output;

You could also create a template for your output then use code to render the template. And another way would be to copy the block_html template to a new template, insert your code, then enter your new template name in the "Template To Use" field.