Masking problem

[RayArt]

Hi Folks, here is my puzzle.

I have an image as a background which I have animated.  I have added a mask to this image.  When I run the slide the mask stays still while the image moves, which is what i want it to do.  But, when I add an image to the Mask Content and run the slide, everything moves including the mask and Mask Content.  Is there a way to stop this happening?  So that when you view the slide the background moves but not the mask and its contents.  Many thanks, Ray