For starters, simply download these images and place it somewhere convenient.
(Excuse for the huge sizes, saves the hassle of needing to rename URLs and stuff for the correct size images)

Right-click on the game in Library view and select and pick the image you just downloaded.


It should look like this once done.

Next is the banner, where the same procedure follows: right-click in the banner area and select Set Custom Banner, then select the banner downloaded.



The icon is a bit more different, after uploading...


...the icon need some adjusting, so right-clicking it and choose Adjust Logo Position.



Simply click on the center green square and resize the image all the way. The image has been aligned to sit precisely at the right side of the banner.

And there you have it. Updated images that makes your Steam Library more organized, Enjoy!