Do you have Flex builder? If so, I should think that you would be able (in design mode) to grab the sides and stretch the image as wide as you like. But then again, I also thought you would be able to do the same thing in the MXML using the Image width
If your app is 600px wide and you want the image at the top, this should work.
<mx:Image x="0" y="0" width="600" source="yourImage.png" />
If that doesn't do it, perhaps some sample code and a more detailed description of what you're trying to do would be helpful.