I think the examples below are self-explanatory. As you can see the buttons containing images are aligned nicely, even if images with different sizes are used. So far I have only tested this page with IE7 where it's rendered as expected. No elegant work-around has been found by me, other than changing the margin for FireFox only. For a cross-browser solution you could make an image with the text, but that doesn't help much if the button text is dynamic.
Example 1