Controlling Layout With Graphics
Because of HTML's limitations with page layout, web designers often use a single-pixel gif to control the look of their pages.
This is simply a single colored pixel that can be set to any size. This is a single-pixel gif at its actual size.
<img src="spacer.gif" height="1" width="1">
At its original size, it's nearly impossible to see. This is the same image, but with width and height increased:
<img src="spacer.gif" height="10" width="10">
Once again:
<img src="spacer.gif" height="40" width="40">