How to make background for slider?

Question

Slider

Good day, comrades. I do not understand how it is possible to implement a gray background for the slider if my images are jpg and their background is white. If you replace them with png, then the slider images overlap each other and do not look very good. Is it possible to do this at all, if so, how? Please note that the right and left have different shades of gray for the background :)

Here is my code:
https://codepen.io/mal48813/pen/pmqdqO