for file uploads to work, the forms encoding type needs to be set to multipart/form-data.
change the form tag:
<form action="articulos_Insert.php" method="post" name="WADAInsertForm" id="WADAInsertForm">
to:
<form action="articulos_Insert.php" method="post" enctype="multipart/form-data" name="WADAInsertForm" id="WADAInsertForm">