Heading text which you can centre or not
You might consider to prepare pics that go on the left to "face" into the right, rather than to be "going off" or "facing off" the page. And the reverse if the pic is on the right. It is something about visual design and there are no hard and fast rules. Now and then it looks well for this rule to be broken. There is a design reason for this: briefly, it is less jarring, more unifying and turns the eye into the text. How you make pic change orientation is a simple matter in almost any image software app. You just flip it along the vertical axis, often this is a simple command. The pic is exactly the same except reversed left to right.
You can see the orientation change and judge for yourself.
After this paragraph will be a clearing div to keep IE6 happy without having to give the main wrapper an actual width (which would have the effect of making the parent box, the one with the border, enclose the floated picture). No need for overflow now.
As for centering the show for IE, it is perhaps not worth the trouble. The easiest way to do this for IE is to give the main box a width, but then one loses some fluidity. One can go to a lot of trouble for little gain. I am unsure what IE7 is doing in this design?