That landscape image is called a grid, and you can change it when that game is the first one in your Recent Games list (the one game showing in grid view instead of the normal poster view). If you right-click the grid image in Recent Games, go to Manage, and then click Set Custom Artwork, you can change the grid image.
Any games you want to do this to, you'll have to start them and then immediately quit them so they're the first game in your Recent Games list. That's the only time grid view shows up for any games now.
Before Steam started doing posters for games, your library was composed of nothing but grids. Nowadays, the only time you see a game's grid is when it's your most recently played game. So there's not really a need to change it, unless it stays at the top of your Recent Games for a long time and it's bugging you.
By the way, you can go to the SteamGridDB to find grids, posters, logos, background images, and icons for every game on Steam. It pulls metadata for every game, then people can upload their own custom artwork and logos. So you can find unique artwork to improve your library. Or add posters to older games that have been abandoned before Steam switched away from grid-only display.
I don't like having games with blank images, so I update every missing poster, background, and logo that I can in my library.