The image data is stored in an array in the options table under the key 'themezee_options'. You need to add the extra image data to this structure when the save settings button is pressed. But it may be worth rethinking the image data structure... hold that thought.
The function displaying the slider is
themezee_display_frontpage_slideshow() (on includes/template-tags.php) and is a pluggable function. If you create a child theme and define this function on the child functions.php, it should be used instead of the original.
If you look at the source code, it's not very efficiently implemented for more than 2 images. You'd be well served to re-design it to work with any number of indexed images instead of 'one' and 'two'. Which means re-designing the way the image data is stored as well, by index instead of 'one' and 'two'. If done properly, any number of images could be added.