You will find the new changes in the Quickstar repo, for background animation: https://gitlab.com/lunafromthemoon/RenJSQuickstart
You can now load a background as a spritesheet, in the setup file as:
backgrounds:The information you need to add is the frame width and height, and the framerate. You can see an example in the quickstart, by changing the background that is loaded in the story file from "room" to "forest".
# key: filepath frame_width frame_height framerate
forest: assets/backgrounds/forest-spritesheet-800x600.png 800 600 5