There are instances where doing so can make sense—namely, when search engines struggle to crawl your site due to technical issues.
This approach, however, isn’t limited to images. It can apply to any type of static content, regardless of how effectively the search engine is crawling your site. For instance, many websites utilize Content Delivery Networks (CDNs) for static content to optimize delivery.
That said, for the majority of websites, separating content into subdomains or domains is largely unnecessary. Not only will it fail to encourage search engine bots to crawl your site more often, but frequent crawling does not equate to a better ranking for your website.
Yes, even if a search engine visits your site regularly, it doesn’t automatically mean your rankings will surpass those of your competitors. Instead, focus on using cookie-less subdomains specifically for static content.
(Source: seroundtable)
SEM MasterPlus: Complex website promotion
