The images you wish to be used in the Slider can be flagged by editing the image in the Media Library itself and setting the Media Categories to Slider Image. It is as simple as that.
On earlier releases of the theme it was 'Main Index Featured Image'. Other categories may also be selected for your own purposes, as in the example.
You also need to tell the Slider which link is to be used when you click on the image in the slider. This is again achieved in the Media Library by setting Linked URLs to ONE of the given choices, which correspond with those in the Links options.
There may be times when you want a particular image to be shown first, perhaps to highlight an upcoming event. There is an option to specify this image by its ID. This doesn't have to be one of the pool of random images.