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.
Your browser stakes out the image area if you set height and width attributes for the image thus avoiding reflows.
Also the bracketed comment makes it sound like this feature doesn't work well?