You can simply use a

kazimmerman — Thu, 29 Jan 2009 13:22:38 +0000

You can simply use a percentage as the width:

...
<img src="..." width="100%" />
...

Or, hopefully you're using CSS:

img {
  width: 100%;
}

The only problem you may see is with Internet Explorer (at least older versions) in that for whatever reason 100% of the screen is really around 98% instead of the full 100 and you may end up with a bit running over horizontally.