Instructions:
Choose an image from the categories below and download it. Then rename it to pic_waitingbkgd.png and copy it into the following directory inside your MoO3 directory: GameDataSets\Classic_01\GameAssets\Common\Graphics\UI\PlayingGame. Create the directory if it doesn't exist yet. Done.

Tip: If you'd like to use more than one background image, download the End-Of-Turn Background Image Rotation Utility and follow the instructions of the readme.