1. A method for testing a graphical user interface comprising: receiving a first image file containing data representing an actual state of at least a portion of the graphical user interface, the actual state including an actual visual appearance of the graphical user interface as it appears when the graphical user interface is displayed on a computer display; receiving a second image file containing data representing a valid state that is intended for the portion of the graphical user interface, the valid state including a valid visual appearance for the portion of the graphical user interface; computing a difference between the first image file containing data representing the actual state of the graphical user interface and the second image file containing data representing the valid state intended for the portion of the graphical user interface based, at least in part, on a weighting function, the weighting function providing weights for parameters of the graphical user interface based on a relative importance of each parameter, wherein computing the difference between the first image file and the second image file includes removing one or more regions of the graphical user interface from consideration through use of the weighting function; comparing the difference between the first image file and the second image file to a threshold value; and determining whether the graphical user interface is valid based, at least in part, on whether the difference between the first image file and the second image file exceeds the threshold value.