Alright, so for work I sometimes have to do lame things like programming HTML e-mails. The amount of effort one puts into getting the result across several mail clients is not what you’d expect.
Anyway, to the point: Google Mail (Gmail) caches all your images. So when updating an e-mail template, don’t wonder about you not seeing the results.
More info at awesome Litmus: https://litmus.com/blog/gmail-adds-image-caching-what-you-need-to-know