If they are Spark registered, then they should be able to click the Advanced Settings option under the Device in the Admin Portal. Then go to Setup>Personalization.
If it's on-prem registered, then just hit the IP address from a browser, login, and do the same process. Setup>Personalization.
With most customers that want to use their logo, they need to create the background in photoshop or gimp, and then make sure the size aligns with the limits that Cisco sets for the wallpaper. (Only BMP, GIF, JPEG and PNG files smaller than 16 megapixels are supported. Custom wallpapers do not apply to touch panels.)