well a cdn is basically a cloud yes. I know there are several ones who support opensource (basically pushing their own google rating up with "natural" links).
but I don't know any for free. you could try cloudflare the free package as a reverse proxy this should help your system if you have load problem.
You could just use nginx or another static lightweight server. so the images will be delivered by it. if you already switch them to another subdomain it's easy to change it to a cdn afterwards. (cdn.mydomain.com) or something