attach image_field to entity in drupal 7

I use drupal 7, and Entity API to develope a module. I have an entity to record client information. I wish to use image_field to let client upload their logo. So I have this function:

    function silver_client_enable()
{
  field_cache_clear();
  field_associate_fields("silver_client");

  if(field_info_field('logo'))
    return;

  $field = array(
    'field_name' => 'logo',
    'cadinality' => 1,
    'type' => 'image',
    );

    field_create_field($field);

  $instance = array(
    'field_name' => 'logo',
    'entity_type' => 'silver_client',
    'bundle' => 'silver_client',
    'label' => 'Logo',
    'description' => 'Logo',
    'display' => array(
      'default' => array('label' => 'hidden')
    ),
    'settings' => array(
      'file_directory' => '/logo',
    ),
    'widget' => array(
      'type' => 'image_image',
     ),
  );

  field_create_instance($instance);
}

In the entity creation/edit form, I use :

field_attach_form('silver_client', $client, $form, $form_state);

to attch the field.

When I called up this form, the image upload field was corrected displayed. An i can use it to uplod file to serve.

In the form submit function, I save the entity as:

entity_save('silver_client', $client);

However, after I press the save button, the entity table is correctly saved. Field table is not. Both field_data_logo and field_revision_logo are empty.

I believer Entity API looks after the retrieving and saving of attached fields. Can someone tell me what is wrong with my code? Thank you.

You have to write the values back into your entity:

field_attach_submit('silver_client', $client, $form, $form_state);
entity_save('silver_client', $client);

http://api.drupal.org/api/drupal/modules!field!field.attach.inc/function/field_attach_submit/7

And you should validate the field values:

field_attach_validate('silver_client', $client, $form, $form_state);

http://api.drupal.org/api/drupal/modules!field!field.attach.inc/function/field_attach_validate/7

Furthermore if you don't want to declare your entity and fields by yourself you might checkout the EntityConstructionKit : http://drupal.org/project/eck which allows to export entity structures with Features just like Views.