This is a problem I'm aware of and I'm slowly solving as I review the content of every page. Design docs had a similar issue, every image was broken for some reason. I didn't find a pattern that would help me understand why a few images broke while others didn't; it seems to be pretty random, actually. Maybe it was a change on how they handle images? They changed a few things internally this month (see https://blog.gitbook.com/changelog).
I've found that following changes through Git isn't reliable at all. They state in their documentation that Git serves just as a mirror; the versioning system they use internally was designed by them. One thing that happened to me earlier this week was that GitBook restored some images I deleted and created copies of some I repeatedly used in the same page. I'm not sure why they did that either. 🤷♂️