DISCLAIMER: ALL IMAGES ARE FREE USE IMAGES TAKEN FROM SCREENSHOTS OF THE STEAM WEB CLIENT, WINDOW 10, OR GOOGLE CHROME BROWSER ON FREE USE TERMS OR ARE DONE WITH PARTITUAL USE IN ACCORDANCE WITH US COPYRIGHT LAW.
When adding a non-steam game or even with some store bought games, the game will appear bland and ugly. To fix it simply open Google, type in the game's name, and click images:
Hit tools --> size --> exactly --> and type in 460 for width and 215 for height.
After hitting Go, choose a picture you would like to be your Steam Banner.
This one looks nice.
Then you right-click and hit 'Save image as...' And simply save the Image
Afterward, go back to Steam and in Desktop Mode and hit the four-squares icon in the top-left.
Then right-click you game and hit 'Set Custom Image...'
Finally hit "BROWSE...,' find your banner, click it and then hit 'Open' and finish it off by hitting 'Set Image.'
Next time you open Steam Big Picture Mode your games will have nice banners!