Image links broken in Gmail because of google's Image proxy

Because your image wasn't open to public.

You need make sure the new Gmail image proxy can fetch your image from google server.

In Google Apps, it's now possible to disable Image Proxy for GMail (in Apps Settings). Or, better, you can white-list your internal URLs for which image proxying should be disabled.

DEG’s research has confirmed reports from wordtothewise blogger, Laura Atkins, that “Google is rewriting image links. This rewriting appears to be happening during the delivery process. Older messages that are currently in mailboxes aren’t showing this tracking.”

For global images, this means only one user has to open the mail and the images are pulled from the server. In the case of tracking images, every image file name is unique. Every new open will cause Google to grab the uniquely named image. The result is that senders can track the first open, but no subsequent opens.”

Reference: http://www.degdigital.com/blog/gmail-tracking-disruption-for-marketers/