Selecting 2 images from a larger selection at random to overlay on top of one another?

So I really know nothing at all about javascript but I'm trying to put something together for a class project and we wanted to have two images (from a larger selection of images) overlapping with the upper layer being slightly transparent so the one underneath could be seen, refreshing either via a button or by refreshing the page. It would also be great if someone could explain if/how you can change the transparency type (I.e. overlay, screen, multiply, etc). Our goal is to make new and interesting random compositions/art using photos of art we've already made. I'm working in cargo collective which has custom HTML and css options. Again, don't really know anything related to coding so feel free to explain as if I'm a 5 year old :)