Featured Slider – alt="" Tag

Hello, How do I enter the alt="" tag for the homepage featured slider images? I tried using the page title already (and hide this of course) but that´s not the right solution: <article class="page pageid-96 hentry slides displayblock"> <figure class="slider-image"> </figure><!-- .slider-image --> The alt="" is still empty ... the picture itself has filled in the text for alterative Tag. Thank you very much in advance.