Hi! If you're using Twine in the browser, I'm afraid I'm of little help as I don't know where exactly to put the image files themselves. For images, make sure they're in the :root, and add a line for "--bg-img: url(images/yourimagenamehere.jpg);". You'll then want to go to your body section and add "background-image:var(--bg-img);".
As for the buttons, this is the code I've been using for my recent buttons, I hope it helps!
button {transition:0.4s; background-color: var(--buttonbg); border: none; color: var(--black); width: 95%; margin-left: 0.25em; text-align: left; font-size: 1.2em;font-family: var(--main-font); letter-spacing:0.1em;}
button:hover {cursor: crosshair; color: var(--accent); text-decoration: none; background-color: var(--black);}
If you need any more help feel free to send me an email or just reply to this comment!