To do this, you will need the Google Cast app on an Android or iOS device that is connected to the same wifi network as your Chromecast. Open the Google Cast app, then tap on the Devices tab. Choose your Chromecast from the list, then tap on the device menu (3 vertical dots), and tap on Edit Background. Here you can choose a custom image or set of images to be used as a background on the Chromecast. Some more information on this can be found here.

Learn how to obtain the Ditto background image containing your connection instructions and get step-by-step setup instructions.