Assuming you use spine-webgl directly like illustrated here:
https://github.com/EsotericSoftware/spine-runtimes/blob/4.0/spine-ts/spine-webgl/example/barebones.html
You can load your background image in loadAssets via:
canvas.assetManager.loadTexture("mybackground.png");
You can then fetch the image texture in initialize() via:
backgroundImage = canvas.assetManger.require("mybackground.png");
And finally, in render(), you can draw the background image as the first thing each frame via:
canvas.renderer.begin();
canvas.renderer.drawTexture(background, x, y, width, height);
// rest of your rendering code
canvas.renderer.end();
