You could've always made a backup of the css file and then when you updated simply renamed it back to the original. That said there won't be an update for a while, when there is the plugin will likely be using a rewritten version of the plugin and I'm still working on an idea where you can easily edit your css from your admin dashboard.
As for the images you could've also simply replaced it with your own one, that's what I've seen many people do, especially easier for those who don't know css. Again though, hopefully you'll be able to easily change the image(s) from the admin dashboard or if one isn't provided, use css instead.
Will mark this as resolved but feel free to open another topic if you have any other questions, problems, etc.