I have observed that even when you set height and width attributes, the area is not always "staked-out." Of course we'd assume that it is. I'll need to get you some browser and version details about this.
Are you saying if you specify <img width="xx" height="xx"> that the browser doesn't reserve exactly the right amount of space for that image? Because that's exactly what it's supposed to do: if it's not, then it's a browser bug.
Update: this must have been my imagination. I confirmed that all of the most common browsers will stake out the image area if height and width is set in the css or the img tag.