When styling your room by going to More > Room Options, there are many places where you can enter a "image url".
This article explains two ways to get an image url to enter into these fields.
In many cases, the image you want to use already exists online, so you just need to get its url.
Watch: Image URLs - 3 Methods of Finding URLs for Images Online
https://www.youtube.com/watch?v=QdzqYYkxyks
However, the above video does not apply to all cases, such as if somebody sent you an image you'd like to use in your virtual room, or if the image does not exist online.
In these cases, you can upload your image to Google Drive.