And when I am trying to choose images for the slideshow, I can not see them in the File manager window (see screen shot). I can see the folder and the number of files in it is specified above. But I cannot see the files.
The add-on of the file should be .gif (and not .jpg like in your case) Change it and try again
First, I enable Builder, create a new block, choose Widgets - Image Slideshow
here, everything is correct