Thank you both for your answers. After posting yesterday I poked around in other skins and had the suspicion that this is specific to Laza's skins.
I got most of it working now but not sure if I am doing it the right way. I had to use
<%= imagePath.replace("thumbs", "slides"); %>
to get the full-size image for the folder I want to link to since I could not find a variable that points to it instead of the thumbnail size.
It's as good a way as any, but you will need to test if the current object is a folder, if not presumably you still want the normal thumbnail image.
When trying to find a variable, or looking how to arrive at a required answer, I always drop in a 'current' statement i.e. ${current}
That will list all of the related variable attributes.