Not sure why an upload button would confuse your users.
Just kidding. I did not realize you would assume you had to use an upload button to use a preloader. I was simply showing something I had on my site. The concept I gave works whether you have a button or not.
Anyway, last night I did an image slider on my site without a plugin and just used a custom state.
I have a group set to align to parent and 3 images inside it, with another group for the arrows. It switches images quickly.
You may want to consider going this route also.
I didn’t have time to optimize the .gif but I think you’ll get the gist of it.