First, right click the wallpaper of the library page.

This will prompt you with a dialogue box containing the option to choose a new wallpaper, selecting to do so will open the file explorer.

Once opened, select the image you wish to replace the original background with. I would recommend a blank or scarcely detailed image that contrasts the game's logo. Like so:

You can return to the original wallpaper at any time by simply right clicking the background and selecting the "Clear Custom Background" option, removing your selected image.

The game logo's location and size can also be altered using options given in the dialogue box mentioned previously.