How can I change or choose Facebook share picture?

Hi, when I share my Homepage on Facebook something strange happens in Simple Catch Pro. The picture that´s automatically shown seems to be my very first blogpost picture. Even though it isn´t on the "home" page and even though I deleted everything from the website and media gallery related to that blogpost. It still shows up when I share the home page on FB. Does it take a while to be deleted completely? I want to choose the best possible picture when someone shares my home page. How can I do that? How can I determine a default picture for social media sharing? When they share one of my blogposts usually the blogpost picture shows up on facebook, which is what I want. But what about the "home" page or other pages... And another question: What is the "Web clip icon" in Theme Options? Is that the default picture that is shared on social media platforms? Thanks for your help!