Does JetPack’s Photon damage Image SEO?
Thus far I haven’t been able to find concrete proof of this but it probably does have an impact. Why is that? Because the URL being served is from a different site than yours which results in Google or other search engines associating that picture to them not you. Obviously there are lots of people out there using CDNs so I believe it is a matter of time until Google and the other search engines recognize this and properly address this issue. In the mean time from what I have been able to gather from other users in the Internet is that there is an impact to the number of visitors and page rank.
My recommendation therefore is that until Google and other search engines are smarter on how they handle CDNs it is best to host it yourself. Also, many CDNs like Microsoft *Windows Azure* allow you to have your own virtual machine which uses the same hard ware they host your files so there is little incentive now to use their network of CDNs unless they are geo-redundant/distributed. (You could also do that yourself but then it becomes a matter of costs).