Well for starters, you have several ID declarations on the same page; you can only use one instance of an ID per page (you can use several different IDs, not used more than once each), so you want to make #postBody a class (.postBody) so you can use it over and over in the same page. Same thing with 'postHeader', anything you are using more than once per page. Whats probably happening is since the <div> element is defined as an ID it breaks out over the structure of other <div> elements, therefore the image breaks out and does its own thing.
Change your CSS, and then <div class="postBody">, etc.
You might consider also changing the postBody definition altogether, make it solely for use with the <div> element, like
div.postBody { somestuff here; }
ID's are great for one single element structure, like <div id="header"> stuff like that.
I also notice in your postBody definition for 'text-align:' is left blank, I would comment it out unless you want to place a value there.
[edit] edited for clarification [/edit]
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