Adding a function to set a custom image via JavaScript with a base64 image string would make it very easy to customize with a local image by just calling a function like setBase64Image(string) with the image formatted as "data:image/jpeg;base64," + data.base64EncodedString() which is set in image.src
I'm currently using this method to customize jQuery WebGL Ripples in a macOS app (Backgrounds) to use it as a wallpaper. With this function it's very easy to use the user's wallpaper or any local file