How I created the original: the image that appears by moving the slider to the right is chopped up into pieces in different states and each state is connected to a part of the slider:
1. I've got about 67 states added to the original image, each state covers 10 more pixels of the new image. So as you're moving from state 1 to state 67, more and more of the image is shown.
2. With 67 states means there are 67 triggers: "the state of the image changes when the slider moves (value is between .. and ..). So as the the slider moves to the right, the new image starts to show up.
The end result looks and works fine, but this process slows down Storyline by a lot (my SL crashed at least 8 times trying to create this) and it's nearly impossible to update the image as the whole process needs to be repeated.