<form role="form" id="contact_form" class="contact-form" method="post" action="{{ path('publier_offre') }}">
 <input type="file" class="form-control" name="" id="imageOffre">
                        </form>

and this the controller action ( where to get the uploaded image and persisting it to the database ) :
 public function publierOffreAction(Request $request)
    {
        if($request->isMethod('POST')){
            $offre = new offre();
            //getting the image file
            $filename = $request->get('imageOffre');
            $filename->move(
                $this->getParameter('image_directory'),$filename
            );
            $offre->setImage($filename);
            //getting the image file
            $em=$this->getDoctrine()->getManager();
            $em->merge($offre);
            $em->flush();
        }
        return $this->render('@offer/Default/publieroffre.html.twig');
    }

/**
     * @var string
     * @AssertNotBlank(message="plz enter an image")
     * @AssertImage()
     * @ORMColumn(name="image", type="string", length=255)
     */
    private $image ;

 /**
     * @return string
     */
    public function getImage()
    {
        return $this->image;
    }

    /**
     * @param string $image
     */
    public function setImage($image)
    {
        $this->image = $image;
    }

parameters:
    locale: en
    image_directory: '%kernel.project_dir%/web/uploads/images'