WordPress doesn’t wrap <image />
tags with <div>
elements by default … so there’s probably something in your theme or a plug-in on your site that’s adding the element wrapper.
I suggest switching to the default TwentyTen theme and disabling plug-ins to compare the generated markup. Then re-enable plug-ins one at a time and switch back to your theme to see which set of code is adding the unwanted <div>
blocks.