Image SEO: alt-text vs focus keyword and meta description

Dear Yoast,

First my compliments for this SEO optimizer, it works well. I however do have a question about Image SEO. I have read the articles at your site regarding this subject but there is still a question unresolved for me.

I have a photo albums on my website in a slideshow, so without supporting surrounding text (albums). I can I fill in the atl-text, caption and desctription. I have my filename tuned to the subject. When I fill this in I can also do SEO as on the pages, like adding focus keyword and meta description.

My question, is it necessary to fill this in as well for better ranking, or is the alt tag sufficient. And when I fill this in, will it have more benefits in the images ranking or will it rank lower as two methods are used. My purpose is to show my images.

Best regards,

Vijay

https://wordpress.org/plugins/wordpress-seo/