Using picture as slider, not only thumb (slider itself invisible)

Hello everyone, 

I'm not very advanced in using Storyline yet, but I hope that my question is not too obvious. I have tried searching but have not found an answer under my keywords. 

I would like to animate a fly swatter as a slider. It should scare away a fly on a window and I would like to use a slider for the movement (but the typical slider should be invisible). I thought this would work because I found a very similar example on YouTube (a hand with a computer mouse moving back and forth on a desk - picture attached. It is a tutorial, but I don't get, how he has done the slider itself). I can certainly solve this differently (e.g. via motion path), but the whole exercise was really just to see how I can create really individual sliders that no longer look like sliders at all. So it's not just about filling the thumb of the slider with an image. 

Does anyone have the crucial hint for me? 

Many thanks in advance!