I haven't done this in GIMP in a long time, but it is easy in Photoshop. So I will tell you how in Photoshop and you could probably easliy figure it out in GIMP.
With the image open use you Marquee tool to select the area you want to remain.
You could use a rounded rectangle as a layer mask in Photoshop.
This is a more contemporary (eg: adjustable/non-destructive) method of doing this.
Not sure if GIMP or Fireworks support layer masks though.