Yes, you only edit one file: themes/yourtheme/theme.php.
If you want to start over, recover the original theme.php from the theme you are using. Then copy the code below into the file, at the very end, just before the last line: ?>
// Display the full size image
function theme_display_fullsize_pic()
{
global $CONFIG, $THEME_DIR, $ALBUM_SET;
global $lang_errors, $lang_fullsize_popup, $lang_charset;
if (isset($_GET['picfile']))
{
if (!GALLERY_ADMIN_MODE) cpg_die(ERROR, $lang_errors['access_denied'], __FILE__, __LINE__);
$picfile = $_GET['picfile'];
$picname = $CONFIG['fullpath'] . $picfile;
$imagesize = @getimagesize($picname);
$imagedata = array('name' => $picfile, 'path' => path2url($picname), 'geometry' => $imagesize[3]);
}
elseif (isset($_GET['pid']))
{
$pid = (int)$_GET['pid'];
$sql = "SELECT * " . "FROM {$CONFIG['TABLE_PICTURES']} " . "WHERE pid='$pid' $ALBUM_SET";
$result = cpg_db_query($sql);
if (!mysql_num_rows($result)) cpg_die(ERROR, $lang_errors['non_exist_ap'], __FILE__, __LINE__);
$row = mysql_fetch_array($result);
$pic_url = get_pic_url($row, 'fullsize');
$geom = 'width="' . $row['pwidth'] . '" height="' . $row['pheight'] . '"';
$imagedata = array('name' => $row['filename'], 'path' => $pic_url, 'geometry' => $geom);
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title><?php echo $CONFIG['gallery_name'] ?>: <?php echo $lang_fullsize_popup['click_to_close'];
?></title>
<meta http-equiv="content-type" content="text/html; charset=<?php echo $CONFIG['charset'] == 'language file' ? $lang_charset : $CONFIG['charset'] ?>" />
<script type="text/javascript" src="scripts.js"></script>
<style type="text/css">
body { margin: 0; padding: 0; background-color: gray; }
img { margin:0; padding:0; border:0; }
#content { margin:0 auto; padding:0; border:0; }
table { border:0; height:100%; width:100%; border-collapse:collapse}
td { vertical-align: middle; text-align:center; }
</style>
</head>
<body>
<script language="JavaScript" type="text/JavaScript">
adjust_popup();
</script>
<table>
<tr>
<td>
<div id="content">
<?php echo '<a href="javascript: window.close()"><img src="'
. htmlspecialchars($imagedata['path']) . '" '
. $imagedata['geometry']
. 'alt="'
. htmlspecialchars($imagedata['name'])
. '" title="'
. htmlspecialchars($imagedata['name'])
. "\n" . $lang_fullsize_popup['click_to_close']
. '" /></a><br />' ."\n";
echo $row['title']."<br />\n";
echo $row['caption']."<br />\n";
?>
</div>
</td>
</tr>
</table>
</body>
</html>
<?php
}
This is the entire function, including the two lines I added to the default function. Once you see that working, you can modify those two lines as desired, making sure to obey PHP syntax rules.