By full width my colleague meant you need to go to Media menu and select the image in question and copy the direct URL to use for the image:
The reason that you need that URL in the slider is, WordPress also generates some Thumbnail and medium width versions of the same image which should not be used.
To install and use Revolution Slider you need to go to X > Overview menu and scroll down till you find the Revolution Slider Extension. There you can install and use it.
For more information:
I also suggest that you check our knowledge base as it will give you a good understanding of the theme and plugins usage:
Finally, if you want to use the Revolution Slider in the Cornerstone, you will have the Classic Revolution Slider element at hand after you create your slider in the Revolution Slider menu and you will see the slide listed in the element.
If you want to use the Page Above/Below Masthead functionality to add the Revolution Slider please read this article for more information: