Also, remembering that Google has no obligation to protect non-American users, does that give the NSA access to them, to run things like facial recognition, etc?
If it's archived {immediately,on first request} permanently thereafter, there's now another copy outwith your control.
Edit: moreover, the images are requested locally by the user's browser. Google doesn't get a look at them at any point[1]. Whereas now, they get a permanent copy for free, because they're doing you such a favour!
[1] Directly, anyway. I guess they could use js to get the requests and headers and submit them back to the mothership, but on slow connections that could be pretty obvious.
(Of course, this doesn't rule out the possibility that Google is actually holding onto each version they cache. I imagine it's not actually worth it for them to do so.)
As comparison, Firefox has a 'save page' function which distinguishes between 'html only' (akin to the mail message), and 'complete', which would include all images, stylesheets, external js files, etc.
The work is obviously to sift through all the emails, remove the photos, and separate/store them on a separate server. Now all NSA needs to do is get a daily dump of all the pics, and then run their data collection/facial recognition on them.