Hi to you Reena_Soni and many thanks for your suggestion. And thanks to the others who replied.
I have solved the problem finally. Plus I have managed to find out how to format the slideshow so that it’s not so huge!!
I just clicked on the Adapt to the image in Settings.
This suddenly made the image look right.
And also I changed the ‘stretch’ of the image from the whole width of the screen to the width of the grid. It’s much better. It’s my own fault for not reading everything and just going straight to panic mode. I hope this helps anyone else who’s struggling with this.