To generate a custom thumbnail for your page, select the landing page of your choice.
Then, click the settings button (bottom left of the icon), or double click the step. After, a popup will appear on the right side of the canvas for the icon.
Next, you're going to need to enter the exact URL of the page that you want to generate the thumbnail for.
Then, just click the Camera button and the image of the page will load for the icon!