you don't need to set a homepage (so don't set a static homepage). WP will automatically use index.php as your mainpage/homepage.
You can add or remove images in 2 ways: either you can edit the header.php file directly (editable area starts at line 89 after following code):
<!-- WANT TO EDIT YOUR SLIDESHOW IMAGES DIRECTLY? ADD ANY NUMBER OF IMAGES YOU LIKE BELOW-->
and add the images there. This is the default option when you first activate the theme.
You can also choose the more advanced version and create a single page that holds all of the images.
Create the page, then upload your images (which should be 900px in width and 240px in height) and add them to the page. Do not add paragrahps, line breaks, text or anything else. Just add the images directly one after another. What you can do is: adding links to these images will work without problems.
Then save the page and head over to the cubit options page. here you will find a dropdown menu called "Cubit Starting Page Image Slider". select the page you just created and save your settings.
This page will automatically excluded from any menu.