They may not be indexed by google at all if you use photon. Photon isn't really a CDN. Sure, you could label it as such but it doesn't do paid CDN features. Photon stores a copy of your image that has been uploaded and requested and caches them. They are served from a repository because of this. Asking "Will it affect rank" depends, if your site is taking 30 seconds to load and this reduces it - you'll get a boost. However, the different URL will probably negatively affect it as well. You can see a plethora of people saying "after activating photon my images got dropped from google", so it's definitely possible. Lots of people complain that none of their photon image are ever indexed. So it may help you based on speed, but for an image I find it unlikely.
Side option: Have you used amazon aws before? They offer a whole year free which you could use cloudfront (awesome CDN) up to 50 gb. If it is a smaller site, you probably won't hit that limit. Even if you go above it, its like .08 cents each gb after. 50 gb a month free for a whole year is a pretty sweet deal really. You can also map the domain name to a cname version to better serve the image.
