Immediately below the Main Page Content area you will find the Optional Elements area. The first thing you will edit in this area is the 'Featured Images" area. Before you edit this you will need to upload the images you need into Cascade if you haven't already.
To select the image you want click on the browse icon to find the image you want. The following will pop up:
If, for example, you know the desired image is in the images folder open the images folder, find the image you want and double click it and then select the Confirm Button.
If you want more than one image, click on the plus sign (+) in the upper left.
Then repeat the process you used for the first picture.