flexible graphic to expand with window

[fourday]

This may be a silly question to most of you, but I'm running a IPS forum website and have used the WYSIWG builder for the main webpage banner logo, along with the 3 column layout template.   It looks great and everything lines up when the browser window is shrunk down a little.  But when the window is expanded, the 3 column become wider than the top graphic which only stretches to a point.  Then it looks terrible.  

How do I make the top graphic stretch to match the 3 column when the window is expanded?  

Appreciate any help you can give.  Thanks.