This is what I had to do since I had the same problem.
1) I selected Edit (under the photo I wanted in the slider)
2) I cropped it down so that it was long and narrow – trying to get it to it to 800 X 300 or depending on the size of your photo a 3:2 ratio.
3) The I clicked on the Crop Icon, and Save (under the photo)
4) And then Update on the right sidebar.
5) If that didn’t work, I went back into Edit in the Photo and selected scale image to 1200 X 500 and again selected Update.
6) and re-viewed the page/slider on the site.
Or try reversing these steps (scale down first/Update, then crop). Eventually it showed up the right size.
Good luck.
