Converting a "common upload field" to one that accepts url links

Hello everyone,

   Have a lil issue I cant seem to figure out.

  I have a basic upload box which works great, but i want a way to pull thumbnails from embedded videos.  well I have figured out how to do this, but the image i need is stored in url form. (http://www.site.com/image.jpg)  My upload field only accepts files already on the computer, not external links to images.

I already have a script that uploads the image and does a ton of stuff to it, and I dont want to edit it much.  My problem is getting the script to accept a external image url, instead of a file location. 

code for the upload field:

script code:   

                 $dbthumb = '';                               if (! empty($_FILES['uploadthumb']['tmp_name'])) {                                    $thumbfilename = str_replace(" ", "_", $_FILES['uploadthumb']['name']) ;                                    $thumbfilename = md5(uniqid(rand())).'_'.$thumbfilename ;                                     $maxsize = 10000 ;                                    if ($_FILES['uploadthumb']['size'] < $maxsize * 1024) {                                         $thumbfilename = str_replace(" ", "_", $_FILES['uploadthumb']['name']) ;                                         $thumbfilename = md5(uniqid(rand())).'_'.$thumbfilename ;                                         //upload the thumbnail if one was specified                                         $maxsize = 10000 ;                                         if ($_FILES['uploadthumb']['size'] < $maxsize * 1024) {                                              $filename = $_FILES['uploadthumb']['name'] ;                                              $filename_without_ext = $filename ;                                              $ext_pos = strrchr($filename, '.') ;                                              if ($ext_pos !== false) {                                                   $filename_without_ext = substr($filename, 0, -strlen($ext_pos)) ;                                              }                                              $ext = strtolower(substr($filename, strrpos($filename, '.') + 1)) ;                //img ext removed here                                              $img_exts = array('nowayhoe') ;                                              if (array_search($ext, $img_exts) !== FALSE) {                                                   if (move_uploaded_file($_FILES['uploadthumb']['tmp_name'], 'uploads/thumbs/'.$thumbfilename)) {                                                        $thumb_name = str_replace("'","",$title);                                                        //createThumb($thumbfilename, getSetting("avatar_h", $db), getSetting("avatar_w", $db), 'uploads/thumbs/', 'uploads/thumbs/', $title.".jpg") ;                                                        createThumb($thumbfilename, getSetting("savatar_h", $db), getSetting("savatar_w", $db), 'uploads/thumbs/', 'uploads/thumbs/', $title.".jpg") ;                                                        $dbthumb = $thumb_name ;                                                   }                                              }                                              else {                                                   $errorresult = "th" ;                                              }                                         }                                    }                               }

basically I want to be able to paste a "http://www.site.com/image.jpg" into the what is not a upload field and have the script pull the image and package it in a way so that it can be used in my current script.

I found a few how tos for uploading external links, such as:

$contents= file_get_contents('http://mydomain.com/folder/image.jpg');

but just not sure how to integrate them into my current script or the upload field, I know i cant change it to text field, so how do I get the link in there?  or can I... confused, everyhitng ive tried hasnt worked so far! 

thanks much for any help you might provide!