If you are wanting your images to skip to the next line instead of stacking next to each other just put in a <br> or <p> after the image tag.
This should all work the same in ns and ie but if you must choose one or the other, gear towards IE.
Specifically, however, gear towards the XHTML 1.0 or HTML 4.0 specs, which, especially if you follow the XHTML 1.0 specs(avail from the W3C), should lead you to having the same appearance in both major browsers.