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.
The change of filenames and path is probably what you did you in.
I switched to a CDN a few months ago. It went smoothly as I moved from: www.domain.com/images/filename.jpg to cdn.domain.com/images/filename.jpg
I did no redirects, but all image links in content were updated to the new URL and the sitemap was also updated. The images are still accessible on the old URL, but Google figured it out on their own and replaced all the indexed images with the CDN version. No loss of traffic.