Animating a photo using slider and motion paths back and forth

I am trying to create a page where a photo moves on various motion paths according to a slider and then it moves back again.

I have created two photos with motion paths attached and I have got as far as making it go back and forwards according to the slider.

The problem I am having is - what if someone goes from one of the steps on the slider backwards instead of forwards as it is only set up to go on the backwards motion path once the forwards one is completed. 

Also as I have two photos I don't want them to display at the same time- but I set them to hide/unhide depending on whether the motion paths have completed then the second photo starts in the wrong location as I reversed the paths in order to get the mirror image of the animation but the starting point remains the same. 

May make more sense if you check out the attached file- any help appreciated!