Magento 1.9 – Fix Call to Undefined Method Collection::getData()

I use (Manthan_Marketplace) extension in my Magento store
I want to add a table to this extension
so create a custom table (photos) in Magento database with name:(manthan_marketplace_photos)
Table photos structure:
user_id…………………img_url
(id of user, primary)………….(image name, text)

and edit config.xml in (app\code\community\Manthan\Marketplace\etc\config.xml):

    <models>
        <marketplace>
            <class>Manthan_Marketplace_Model</class>
            <resourceModel>marketplace_mysql4</resourceModel>
        </marketplace>
        <marketplace_mysql4>
            <class>Manthan_Marketplace_Model_Mysql4</class>
            <entities>
                <seller>
                    <table>manthan_marketplace_seller</table>
                </seller>
                <photos>
                   <table>manthan_marketplace_photos</table><= I add this code 
                </photos>
                <vendorproduct>
                    <table>manthan_marketplace_vendorproduct</table>
                </vendorproduct>
                <review>
                    <table>manthan_marketplace_review</table>
                </review>
                <rating>
                    <table>manthan_marketplace_rating</table>
                </rating>
                <sellerrate>
                    <table>manthan_marketplace_seller_rated</table>
                </sellerrate>
                <payment>
                    <table>manthan_marketplace_payment</table>
                </payment>
            </entities>
        </marketplace_mysql4>
    </models>
    <resources>
        <marketplace_setup>
            <setup>
                <module>Manthan_Marketplace</module>
            </setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </marketplace_setup>
        <marketplace_write>
            <connection>
                <use>core_write</use>
            </connection>
        </marketplace_write>
        <marketplace_read>
            <connection>
                <use>core_read</use>
            </connection>
        </marketplace_read>
    </resources>..................some more code  

Then I create a Photos.php in (app\code\community\Manthan\Marketplace\Model):

class Manthan_Marketplace_Model_Photos extends Mage_Core_Model_Abstract {  
        protected function _construct() {
        $this->_init("marketplace/photos");
    }

}

and create Photos.php in(app\code\community\Manthan\Marketplace\Model\Mysql4):

class Manthan_Marketplace_Model_Mysql4_Photos extends Mage_Core_Model_Resource_Db_Abstract {
    protected function _construct() {
        $this->_init('marketplace/photos', 'user_id');
    }

}

and after that create Collection.php in(app\code\community\Manthan\Marketplace\Model\Mysql4\Photos):

class Manthan_Marketplace_Model_Mysql4_Photos_Collection extends Mage_Core_Model_Resource_Db_Abstract {
    public function _construct() {
        $this->_init('marketplace/photos');
    }

}

now when i use tis code :

$img = Mage::getModel('marketplace/photos');
   var_dump($img);  

it return:

object(Manthan_Marketplace_Model_Photos)#60 (15) { ["_eventPrefix":protected]=> string(13) "core_abstract" ["_eventObject":protected]=> string(6) "object" ["_resourceName":protected]=> string(18) "marketplace/photos" ["_resource":protected]=> NULL ["_resourceCollectionName":protected]=> string(29) "marketplace/photos_collection" ["_cacheTag":protected]=> bool(false) ["_dataSaveAllowed":protected]=> bool(true) ["_isObjectNew":protected]=> NULL ["_data":protected]=> array(0) { } ["_hasDataChanges":protected]=> bool(false) ["_origData":protected]=> NULL ["_idFieldName":protected]=> NULL ["_isDeleted":protected]=> bool(false) ["_oldFieldsMap":protected]=> array(0) { } ["_syncFieldsMap":protected]=> array(0) { } }

but when i use this code:

 $result=$img->getCollection()->getData();  

return:

Call to undefined method Manthan_Marketplace_Model_Mysql4_Photos_Collection::getData()

why getData() is undefined for this one but is ok for another model?
thanks a lot for your help!!