I wouldn't recommend using a 'middle' property of the align attribute for image elements for several reasons:
* it does a _vertical_ alignment of an image (with the current baseline) -- and the OP, if I understood them correctly, wants an image to be centered relative to the left and right column margins, i.e. horizontally.
* it's a 'deprecated' attribute -- means it won't be supported in the future, so next generation ob browsers may render such page by another way.
that "visual" editor does center a _selected_ image when an 'Align Center' button pressed on its toolbar, using exactly the method described above by kimik0, i.e. it places an img into Paragraph element with an inline STYLE="text-align: center".
[me wanders why doesn't it add an apropriate 'float:*' to the image instead of deprecated ALIGN for left/right flushed images]
however, I wouldn't also use this feature as centered images are generaly good if they are big enough to be used as a 'pool-outs' across 2 or 3 columns of text. since here we can use only single column layout, centered images which are narrow than 90% column width will look rather ugly -- I don't know how to make text wrapped around centered images within a single column.
finally, just for the record: W3C recommended way to center an inline object (like image) is
style="display: block; margin-left: auto; margin-right: auto" which is also approved by the WP Codex as using a "text-align: center" may fail under some conditions.