Image in post not showing correct

Hello Alex,
Is there I need to add same image in post content and in featured image or only I add featured image it will show in content of single post? currently my featured image not showing in content so I have added it manually in post and it is not showing well like as showing in demo:
https://kerryseo.co.uk/facebook-the-concern-of-currently-being-hacked-and-attacked/

please advice.
thanks