In reply to apachx.

This CDN is only for open source assets. Images from media library are not open source, while most/all JavaScript and CSS and assets referenced from them (images, fonts…) are.

Note that you can use other CDN as you would normally do in combination with commonWP for images or other assets. For free solution for images in media library, there is CDN by Jetpack Photon but it has condition that you must use Jetpack and connect site to their service.

Using CDN for images in media library requires more caution (see comment by Diogene). For new visitors, CSS/JavaScript files are more important because it’s the first thing browser requests when loading page and they affect page’s rendering time. Images are referenced later in HTML, are usually below the fold, maybe even lazy loading is used, so priority and effect of performance is lower. For high trafficked sites that rely on media content, CDN for media library images is important too.