Smplest way might be that if their id number is 23 and you have an image called "23.jpg" in an images folder, then display it.
If your user has uploaded a file and you moved it to the images folder, and assigned it their default user id, and it is found, then display it.
$id = 23 ; // from somewhere ...
// later, when outputting their record
if( file_exists( '/images/' . $id . '.jpg' )
echo '<img src="/images/' . $id . '.jpg" />' . PHP_EOL ;
Thats a quick and dirty method which will work fine, and you don't need to frig with your database.
You might want to flag its existence in the dbase later, or get an alt text, or a title or an upload date, or have more than one photo .... later eh?