Affects of multiple subdomains on homebrew CDN for images

[cat5com]

We're creating our own CDN such that instead of serving images from

http://mydomain.com/images/shoe.jpg

It will appear at all of the following subdomains:

http://cdn1.mydomain.com/images/shoe.jpg

http://cdn2.mydomain.com/images/shoe.jpg

http://cdn3.mydomain.com/images/shoe.jpg

http://cdn4.mydomain.com/images/shoe.jpg

Image tags on our pages will randomly choose any subdomain for the src. The thought was this will make page loading faster by paralellizing requests across many cookie-less domains.

How does this affect :

-Ranking of images on Google image search.

-Ranking of pages they appear on

-Domain authority (images are linked to heavily in our social media efforts, so we will 301 redirect image urls to cdn1.mydomain.com)

Should we disallow all but one CDN domain in robots.txt? Will robots.txt on an image only subdomain even be retrieved? Should we just use 1 CDN subdomain instead?