how to get images aligned properly in editor

I’m using WP2.1 . I’m trying to make sure that the text falls below the image rather than next to it. Irregardless if I align the image left, right, top or whatever, the text still flows next to it rather than below it.

What do I need to do to achieve this? is it a css issue? If so, what should I be looking to do?

Possible plugin? though, I prefer to avoid plugins .