I'm commenting on this way way too late for anybody to read it except for jasonfarnon, but:

Doesn't gmail prefetch / proxy img srcs, in order to not give away tracking/open information to the sender? Or is that something they did a while back but then turned off... Anyway, it would be so lovely when you do a takeout of your gmail data, if they could give you both the original and also a version that had the src rewritten to either a base64-encoded copy of the image or a local file that's included in the takeout dump.