Need a proper thumbnail to show on Linkedin and twitter

I have a client who has a couple of sites that she wants to share on social media but they aren't showing proper thumbnails it shows a fuzzy logo from the website [login to view URL] but it needs to show a nature scene from the slider [login to view URL] I've tried various plugins in wordpress but nothing seems to work. I've tried setting a featured image but this doesn't work either. I have another site where I have the same problem [login to view URL] again shows up blank on linkedin [login to view URL] again on wordpress. I need someone who can sort out a way of showing a picture we choose on either Twitter or Linkedin.