I found a solution, but it is not very elegant:
Add "Activate mouseover image for thumbnails" in the "lay options". Upload a transparent .png file and use it as the thumbnail. For the mouseoverstate choose the image you actually want users to see.
Problem with this is that the transparent .png needs to be the same dimensions then the mouseover picture to fully portray it. Also it appears when you are anywhere inside the container not just over the text.
Armin, how do I address the specific thumbnail image over JavaScript?
My idea was to display:none; the thumbnail image and write a function to show it somewhere random in the viewport.