Ok,
Two places actually: 
You’ll need to just strip out the “img” html from the generation process
NOTE: this would effect ALL functionality of this across your website, including the CP.
(re-iterating for reference for other visitors)
In cp.publish.php in ExpressionEngine 1.6.x, around line 9833:
Change:
if ($data['is_image'] == 1)
{
$imgsrc = '<img src="'.$file_url.'"'.$properties;
$wh = '';
if (function_exists('getimagesize'))
{
$imgdim = @getimagesize($query->row['server_path'].$data['file_name']);
if (is_array($imgdim))
{
$imgsrc .= " width=\"".$imgdim['0']."\" height=\"".$imgdim['1']."\"";
$wh = "width=".($imgdim['0']+15).",height=".($imgdim['1']+15).",";
}
if (isset($data['orig_name']) AND $data['orig_name'] != '')
{
$imgdim = @getimagesize($query->row['server_path'].$data['orig_name']);
if (is_array($imgdim))
{
$wh = "width=".($imgdim['0']+15).",height=".($imgdim['1']+15).",";
}
}
}
$imgsrc .= " />";
To:
if ($data['is_image'] == 1)
{
$imgsrc = ''.$file_url.''.$properties;
$wh = '';
if (function_exists('getimagesize'))
{
$imgdim = @getimagesize($query->row['server_path'].$data['file_name']);
if (is_array($imgdim))
{
$imgsrc .= "";
$wh = "width=".($imgdim['0']+15).",height=".($imgdim['1']+15).",";
}
if (isset($data['orig_name']) AND $data['orig_name'] != '')
{
$imgdim = @getimagesize($query->row['server_path'].$data['orig_name']);
if (is_array($imgdim))
{
$wh = "width=".($imgdim['0']+15).",height=".($imgdim['1']+15).",";
}
}
}
$imgsrc .= "";
In cp.publish.php in ExpressionEngine 1.6.x, around line 9894:
Change:
function fileplacer()
{
if (document.upload.style[1].checked)
{
var file = '<?php echo $file_url; ?>';
}
else if (document.upload.nonimage.value == 'yes')
{
var file = '<?php echo $props; ?>';
}
else
{
if (document.upload.style[0].checked)
{
var file = '<?php echo $props; ?>';
}
else if (document.upload.style[2].checked)
{
var file = '<?php echo $popup_link; ?>';
}
else
{
var file = '<?php echo $popup_thumb; ?>';
}
}
To:
function fileplacer()
{
if (document.upload.style[0].checked)
{
var file = '<?php echo $file_url; ?>';
}
else if (document.upload.nonimage.value == 'yes')
{
var file = '<?php echo $props; ?>';
}
else
{
if (document.upload.style[0].checked)
{
var file = '<?php echo $props; ?>';
}
else if (document.upload.style[2].checked)
{
var file = '<?php echo $popup_link; ?>';
}
else
{
var file = '<?php echo $popup_thumb; ?>';
}
}
(notice the first document.upload.style[1].checked changes to document.upload.style[0].checked - notice the 0 instead of 1)
