How to create multiple slideshows on my front page?
I would recommend using Views Slideshow as it is a very easy to install and configurable module if you are used to using Views in Drupal.

All you would have to do then is intwine your logic, looping through users and their associated uploaded images and place the block on your front page.

Here is a tutorial for drupal 8 and drupal 7 using views slideshow: 

I hope this helps.