Slideshow overlapping on edges

Site URL: https://www.pushplusminus.com/ontography-madrid

I am using a slideshow with a medium-size image on the page. All the images are of the same dimensions: 2500x1875, a 1 : 1.333 ratio. When I use the arrow to go from image to image, the edge of the previous image shows for a second or two. How can I correct this glitch? Is there CSS I can add, or some other way to specify the dimensions of the container?

Thank you.

Add to Home > Design > Custom CSS

.gallery-slideshow-item-img {
    transition: none !important;
}