In Graphene Options >> General >> Slider Options, look for the box marked “Slider image” and select “first image in post” in the drop down menu, and it should pick up the first image to display in the slider.
However, it looks like you are linking to images that you do not host. These will not show up in the slider. For an image to show up in the slider, you must upload it when writing the post, or choose an image that is in your media library.
Ken