I recently red multiple blogs entries on methods of hosting images at subdomain at wordpress. Personally I believe that someone thought very narrowly about todays so fats internet browser. the arguments in support of subdomain was that, browsers make connections to download resources of site from servers. so from one DNS they limit the connections, where as using multiple DNS will give more connections to browser and eventually site will load fast. However; I totally reject this argument. first of all browsers we use today are the fastest than ever before. 2ndly the search engines ask web developers to reduce the DNS lookups for sake of SEO and load time. According to big G: I tested my blog with both images at subdomain and without it. But I found the image without subdomain were reducing the DNS lookups and there was no difference in speed.
Never heard about this but I can't see how it would speed things up. Using a Content Delivery Network or a service like Cloudflare works to speed up serving images. I personally prefer Cloudflare because it is easier to setup and cheaper.
CDN are good of sites having tons of pictures or with millions of pageviews perday. Im not very much worried about my speed. because I've 99% speed and google page speed and 95% at yslow