Thank you. I have added a height in css code to the already existing class, and it works now. All the pictures, small and large, have now the same height, which is ok for me.
But I have another question:
Is it possible to have on one page two slide shows running at the same time? I have tried it with a div tag around each list of pictures, and do have two slide shows then, but the second starts running after the first has stopped.