As you may know, you can add a small image in the right margin of a post, as I do on my blog, by using the Set image command in the popup menu for the post. 

When you choose the command, enter the URL of a smallish image, usually not more than 150 pixels wide. The image will be displayed in the right margin.

This post has an image of a racehorse in the right margin.

Update 4/14/16: I added a small feature in v0.88m that allows images to flow through the Twitter and Facebook metadata. 

Unfortunately Facebook will reject the image if it's less than 200 pixels wide, which makes this not very useful. 

Twitter will include the image, and scale it to fit the space it has. It won't look beautiful, but it's better than having no picture at all.

If you don't want this feature, simply edit the template for your site and remove the image inclusion tags.