Main page sliding banner consist of 5 HTML banners that you can locate in banner manager section. You can add more banners of cause.
Take "Main Banner 1" as example.
If you open banner to edit, you will see HTML source code that you can edit to include your own image and text.
Default images is located in storefront/view/default_html5/image/banner_image_1.png (600x300)
You can upload image to the same directory or any other web directory. It can even be in different server. In this case you use full domain based path to your image src.
To make sure your image fits the banner you can create image with the size that works the best for your specific case.
If you need to change design for each slide, you can edit CSS: storefront/view/default_html5/stylesheet/onebyone.css
You can also include css override right in the banner HTML
I think I covered this in detail.