The main slider is made by the ANG Property Slideshow widget in fullscreen position. It has flexible settings and display images from the listing posts, what you specified in the widget setting.
To change the image, edit or add the property posts.
If you want to display separate images in the slider, you can use other widget, for example ANG Mega Slideshow It can use different posts types. Just create a separate category of posts for the slider, then create posts with images and display via ANG Mega Slideshow.