Theme customizer

Hi, which theme are you trying to use?
You can usually find the images in a particular widget or page.
If you can be more specific, I would be happy to help you out.

You will need to create a slider to replace the demo slider used in the settings under Customize->Front Page.
To create a slider, you will need to add the images to your Media Library and edit the ones that you want in the slider (you will be able to create the slider when you edit the image).

Please let me know if you require any further assistance.

there a video on this link showing how to modify the slider. good luck
http://www.themesandco.com/creating-a-slider-with-the-customizr-wordpress-theme/