This may be a stupid question but is it possible for all emails to default to "Display Images" without having to click the button for every message?
I don't think there is. It does it for security reasons I believe. I've only seen that setting able to be changed through an apk file that was modified by a dev a while back. Otherwise, I don't think there's anything you can do on the stock app.
Is that .apk readily available and would it play nice with the One?
Security or to save people from eating up their data?
You get that on this phone too, one click in any given email to show the pictures in that specific email, and then when you click that, it changes to an option to always show pictures from that sender.The 4.2.2 version of the vanilla gmail client on my nexus 4 gives you an "always show picture from sender" option that will do this for you, so maybe when the HTC One is updated to that version of jb it will have it as well?
You get that on this phone too, one click in any given email to show the pictures in that specific email, and then when you click that, it changes to an option to always show pictures from that sender.