These are custom images you can use to represent games in your Steam library (including non-Steam shortcuts). They will be visible in “Grid View” and “Big Picture Mode”. Customizing your grid images lets you make your library as beautiful as it can be!
My library, also using the Pixelvision 2 skin for Steam itself.
To install an image, switch your Library to “Grid View” (it’s in the upper right corner of your library screen). Then right-click on the game and select “Set Custom Image” from the pop-up menu.
Now use the pop-up window to select the custom image on your hard drive. It may default to showing only .jpg images, so if you are using .png images like mine (they are higher quality) you might have to tell Windows to show them when you browse your drive for the image.
And *poof* it’s customized!
If you want to remove the image, right click on the game and select “Remove Custom Image”. This will return the game to the original default image. You never have to worry about “losing” those, you can always switch back.
You can find more detailed information about using and making custom images in my blog on Destructoid.