Recently we switched over to a CDN for all our images. CDN.example.com, for instance.
We've seen our Image search traffic for sub.example.com get completely crushed to zero. We used to get hundreds of clicks and thousands of impressions a day.
Before the CDN, images had sometimes come from other subdomains, other domains, or our main WWW. They all performed well. Now all come from CND.example.com, and sub.example.com's image search traffic has disappeared.
The filenames changed to be long strings of gibberish, but the ALT tags remained unchanged.
Any idea why this might have happened?