Automated Slideshow

Using feh

sudo apt install feh
feh -rzsZFD 5 /home/my_folder

So here is what you do:

Make a directory with only your pictures in them.
open a terminal window and change to the directory made in the previous step.
Run this command, substituting 5 with however many seconds you want to wait in between updates.

while true ; do for i in * ; do cp "$i" ~/slideshow.jpg ; echo $i ; sleep 5 ; done ; done

Open gthumb and view the image in your home directory called slideshow.jpg, it should rotate the picture. You can make gthumb full screen now if you need to.
Try adding and removing pictures from the picture directory. It won't actually update until its finished going through the whole directory each time, but it will update. The echo $i is in there just so you can see what picture it just copied into place each time.
When you are ready to stop the slideshow, you can press Ctrl-C in the terminal window.