if(is_admin($admin)) {
        echo "
        <b>"._RIMAGEFILE.":</b><br>
        <input type=\"text\" name=\"cover\" size=\"40\" maxlength=\"100\"><br>
        <i>"._RIMAGEFILEREQ."</i><br><br>
        ";
    }
Replace it with:
    if(is_admin($admin)) {
        echo "
        <b>"._RIMAGEFILE.":</b><br>
        echo "<select name="cover">";
        $path1 = explode ("/", "images/reviews/");
        $path = "$path1[0]/$path1[1]";
        $handle=opendir($path);
        while ($file = readdir($handle)) {
          if ( (ereg("^([_0-9a-zA-Z]+)([.]{1})([_0-9a-zA-Z]{3})$",$file)) 
            AND $file != "AllTopics.gif") {
            $tlist .= "$file ";
          }
        }
        closedir($handle);
        $tlist = explode(" ", $tlist);
        sort($tlist);
        for ($i=0; $i < sizeof($tlist); $i++) {
          if($tlist[$i]!="") {
            if ($topicimage == $tlist[$i]) {
              $sel = "selected";
            } else {
              $sel = "";
            }
            echo "<option name="cover" value="$tlist[$i]" $sel>$tlist[$i]n";
          }
        }
        echo "</select><br>";
        echo "<i>"._RIMAGEFILEREQ."</i><br><br>
        ";
    }