Hello Casper,

You can achieve such functionality by creating a new converter for the ButtonTool and change the Image depending on the IsEnabled property of the control.

For example you can try the following snippet that will convert the state:

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if ((bool)value)
                return new Uri("tick.png", UriKind.Relative);
            else
                return new Uri("cross.png", UriKind.Relative);
        }

For additional reference please refer to the attached sample.  

If you need any further assistance with this matter please feel free to ask.