When image optimization is on, Autoptimize will look for png, gif, jpeg (.jpg) files in image tags and in your CSS files that are loaded from your own domain and change the src (source) to the ShortPixel CDN for those. Important: this can only work for publicly available images, otherwise the image optimization proxy will not be able to get the image to optimize it, so firewalls or proxies or password protection or even hotlinking-prevention might break image optimization.