Having a width in there makes no sense. If you change the width or for that matter change the picture then you have to back into the CSS to change it.
Another way to do it...
<style type="text/css">
.centeredImage
{
text-align:center;
display:block;
}
</style>
<img src="imgName.gif" class="centeredImage" alt="image description" height="100" width="100">
Remember to use class and not id since you are using . and not #.
There are even more ways to do it. Remember what you are looking to do is to put what you want into a block and then allow CSS to manipulate that block... if you need padding etc... it is to the BLOCK not to the IMAGE. What the image is should have no effect on what is going on with the CSS... ie. using width.
<width> and <center> are depreciated tags which makes us think differently since with those we were working with the image itself not with the block it was enclosed in.