A: Hi sumo-lingo, great questions!
1. If you use the Easy IO CDN (which we highly recommend), yes the compressed images will be delivered from our CDN using one of our domains. However, the jury is still out on whether or not that actually impacts SEO. A guy that works at Google says it doesn't, a bunch of folks that don't think it does... At any rate, we do not use a custom domain name at ewww.io, and it has never affected our SEO.
Additionally, we include a "canonical" header for all images that points search engines back to your server for the "real" authoritative version of your images.
2. As noted in the feature list, you can indeed use a custom domain for the CDN, and that's what some recommend to help with SEO when delivering images from a CDN. Primarily, using your own domain allows you to keep tabs on your image search stuff better with Google Search Console.