No featured thumbnail issue on Facebook share of homepage

*This only concerns the homepage - all other pages share with a thumbnail perfectly. In searching the forum here I saw you mentioned adding SEO by Yoast to solve the problem with no thumbnail showing on facebook. I'm very new at this - if I download and activate SEO by Yoast and I already have SEO Keyword Page will there be any conflict? If I need to uninstall SEO Keyword page and activate SEO by Yoast - will I have any problems with my website? Thank you so much for you help. http://his-israel.com