How to fade out sides of images?

[JohnM]

I have a gallery same as >>> Link.When you make the browser wider, you will notice that the right and left side of images is fading out in black.I need to apply the same feature on my gallery but have no idea. I have found the this >> link as well but its just a horizontal line not sure how to attach it to both side of images and make the same result as the link.In the comments, ultranaut mentioned that I can apply the filter on images but the question is that if I apply it on the images how to adjust the size, because browser windows might be in different size and the pictures side should be adjustable to every browser size.