How to use downloaded images, for specific field?
You need to get the file fid; you can use EntityFieldQuery.
Then insert/update the "file_managed" table, using code similar to the following one:

db_update('file_usage')
  -&gt;fields(array('id' =&gt; $user-&gt;uid,))  
  -&gt;condition('fid', $file-&gt;fid)  
  -&gt;execute();