Well, this is where you set the name of the file being saved:
$target_path = "upload/";
$target_path = $target_path . basename( $_FILES['picture']['name']);
In this case, you're building the file name in the variable $target_path
. Just change that to something else. What you change it to is up to you. For example, if you don't care about the name of the file and just want it to always be unique, you could create a GUID or a Unique ID and use that as the file name. Something like:
$target_path = "upload/";
$target_path = $target_path . uniqid();
Note that this would essentially throw away the existing name of the file and replace it entirely. If you want to keep the original name, such as for display purposes on the web page, you can store that in the database as well.