Hello @EcomLove ,
you forgot to share link on post.
You can add slide show by cutomize theme from shopify admin and then add section for slide show where you want to add.
You have to go to header file and find the this code '<div class="announcement-bar">'
under this code there is <p class="announcement-bar__message"> and </p> this code.
Replace <p> with <marquee></marquee> like this