I prefer images not being centered by default, they should adjust the way the reader prefers to read text (left to right, or vice versa). But that’s really always a personal flavor.
Markdown itself doesn’t have such capabilities, you’ll always need to use HTML for such things. Similar thing for showing a fixed scaled image size.
It might be a nice addition to add alignment icons into the editor, similar to what Wordpress allows, and just do the HTML magic there.