Step 1: In the AIO Console go to Setup > Banner > HTML5 Slideshow.
Step 2: Find the slideshow that is active for your website and click "Click here to edit the HTML5 Slides"
Step 3: Take note of the existing slide proportions.
Step 4: Click [Add Slide]
Step 5: Choose your file, set the picture width equal to your current slides width, then submit.