Thanks both of you!!! I tried both solutions - the droplet from BlackBird and the solution from DarkViper. Both of them work, and are quite fast. Interestingly enough the solution from BlackBird always uses the fg-thumbs folders (and thus produces thumbnails), while DarkViper's version uses the bigger versions of the pictures.
But - I only can get ONE random picture this way. If I call the same droplet 2 or 3 times, the same random picture will display 2 or 3 times. I guess in order to have 2 or 3 DIFFERENT random pictures, I need to create 3 almost identical droplets, but with different names and then use those...?
Also, I need to figure out how to extract part of the URL to display as text underneath those pictures, so that it indicates from which year and which party in that year these pictures are from. And possibly even link the image to the foldergallery of that album...