To put it simple Display:block helps images look the same in both ie and firefox. So anytime you can't get image to look the same in ie and firefox i strongly recommend applying display:block something like this.
<img src="the image" style="display:block;">
- The best results i have noticed is having it directly on the image but sometimes you can wrap the image in a div and put display:block in that div and that will work well to.