Just append ".ashx" to the end of the image filename to get a dynamically resized version.

            <img src="images/iguana.jpg.ashx" />    No Change
            <img src="images/iguana.jpg.ashx?h=100" />    Resize Height only (width is scaled)
            <img src="images/iguana.jpg.ashx?w=100" />    Resize Width only (height is scaled)
            <img src="images/iguana.jpg.ashx?h=100&w=100" />  Resize Height and width