Remove the "height" property.
<img src="big_image.jpg" width="900" alt=""/>
By specifying both you are changing the aspect ratio of the image. Just setting one will resize but preserve the aspect ratio.
Optionally, to restrict oversizings:
<img src="big_image.jpg" width="900" alt="" style="max-width:500px; height:auto; max-height:600px;"/>