Is it possible to change the order of the images to RANDOM, so the visitor gets another image each time the website is visited?

And what's the maximum number of pictures that can be uploaded/shown?

Please edit below file
modules\mod_btslideshow\assets\skitter\js\jquery.skitter.min.js

Find
show_randomly:false
and change it to
show_randomly:true

you can upload as much as you want. but it could make you site run slow down. I think 5-7 images is good