Managing the featured image on blog posts

Hi,
I have a basic query about how the featured image works for blog posts. It’s something that I have never really given much attention to there are one or two things confusing me.

1) On my blog I have always placed a small image of 300 px on longest edge at the top left of the blog post and then used the ‘more’ tag after 150 words so as to render a snippet of the post on the blog page. I have noticed that most blogs don’t do this with the image. How do they tell wordpress which photo to render on the blog page? I want to create some posts where an image will not be placed on the top left of the post, and I’m unsure as to how to go about it.

2) Is this different from the sharing image which social media plugin will use such as addthis, for facebook shares and the like?

Thank you very much for any help