Add link to my module output
With the few things I know about your field field_surl and about the way you tried to solve it, I might purpose this simple answer, sorry if it sounds obvious :

You wrote :

$output .= '&lt;li&gt;&lt;a href="'.$field_surl.'"&gt;&lt;img src="'.$img_url.'" alt="'.$sld-&gt;title.'" /&gt;&lt;/a&gt;&lt;/li&gt;';


Where did you initialize your $field_surl variable ? At least, assuming $field_surl is loaded within the same node, try :

$output .= '&lt;li&gt;&lt;a href="'.$nd-&gt;field_surl.'"&gt;&lt;img src="'.$img_url.'" alt="'.$sld-&gt;title.'" /&gt;&lt;/a&gt;&lt;/li&gt;';


Or even better, as suggested in @Brandon comment, use url() function :

$options = array('absolute'=&gt;TRUE); //i.e, or set whatever options you need for your url
$my_url = url($nd-&gt;field_surl, $options);
$output .= '&lt;li&gt;&lt;a href="'.$my_url.'"&gt;&lt;img src="'.$img_url.'" alt="'.$sld-&gt;title.'" /&gt;&lt;/a&gt;&lt;/li&gt;';