There is a cut and paste bug in GUIAnimation.cs :

_images[index].MaskFileName = _maskFileName;
_images[index].FlipY = _flipX;
_images[index].FlipY = _flipY;

should be changed to
       
_images[index].MaskFileName = _maskFileName;
_images[index].FlipX = _flipX;
_images[index].FlipY = _flipY;

h4. Steps to Reproduce
in myHome.xml :
<type>menu</type>
what I did was something like this:
<hover flipX = "true" diffuse = "Thumb_Mask.png"></hover>
which did not work.