Update wpuf-functions.php

Temporary fix for a problem with adding images using wysywig 'add-media' button. 

Images added to the post are associated with the add-post-page as well as the submitted post so they appear under the add-post form for all users. I just manually entered my add-post page id; it would need to be manually adjusted for different sites.
If there isn't a better solution, I'll modify the code to automatically get the id of the add-post page.