Every time that I take a screenshot using the native Take Screenshot tool it comes up something like this...

As I said in the title I'm running Ubuntu 9.04 on a Dell Inspiron 600m laptop with an external monitor as a second screen. When I take the screenshot on the laptop monitor it comes out as above, but when I take it on the external monitor the black bar only extends across 3/5 of the image. What could possibly be causing this annoyance?