You are probably using a flexible width theme, and if you insert images individually and try to place them exactly, then funny things may happen when the window size is adjusted.
I highly recommend using a gallery if that works for you. See http://en.support.wordpress.com/images/gallery/ for more information. Note that you can add a single column using all large images if you add the following to your post while you are in html editing mode:
[gallery size="large" columns="1"]
Looks like you've been making some other adjustments to your CSS! If you have more questions about that, please start a new thread in the CSS forums and we'll help you make some more updates. :) http://en.forums.wordpress.com/forum/css-customization/