How to add missing alt tags in a web page designed with visual composer?

Hi,there I am running my site Reset Addiction on wordpress selfhosted. I have got it designed by my web guy.My SEO person recommended me to missing alt tags in the webpages,I do not know much about coding and designing, but I see that he said that my website is designed using visual composer, where can I find options to add missing alt tags to slideshow on home page and other images. Any resource on how to do it will be helpful for me to work on it. Thank you