Lets say a page on example.com has all of its images coming from img.example.com:
<img src="http://img.example.com/keyword.jpg" alt="keyword"/>
while robots.txt on img.example.com disallows crawling:
User-agent: *
Disallow: /
Would this make such images contribute less to a keyword value comparing to images from allowed domains?
I would guess so, because google probably would want to crawl an image url to see if anyone is using it on other sites, or if it even exists.
If that would be the case CDN server would be a fatal mistake for SEO. As long as you use the Alt and Title attributes it shouldn't be a problem, the only problem might be trying to rank an image on google images from this servers.
Actually, that prohibition will affect SEO for Google Image Search, not Google Web Search.
google probably would want to crawl an image url to see if anyone is using it
No, it won't affect page ranking.
P.S. Sometimes, crawlers ignore these rules in order to check your site for black-hat SEO and on other reasons, so don't treat it as reliable way of protecting your images from being accessed directly.
My research is similar to Christofian - in that the alt tag (and don't forget the title tag) is used to describe the image to the search engine. However, since the ALT is on the domain which is searchable, then use it - it will work as expected.
However, it does one more thing which is alter the content:code ratio - I won't go on about that now as it's outside scope of this conversation.
