Image alignment options vary depending on the size of the image you've uploaded. You will only see the options appropriate for that image dimension.
If sufficiently large, default image placement scales the image dimension to the width of the text column body of your story.
If the image isn't wide enough to fill the whole column, it will be centered, as below:
Left-alignment in the image toolbar causes the body text to flow around your image:
Sufficiently large images set to wide will exceed the text column of your story:
Images which are large enough may be set to full-width which will scale to the width of the browser window.