[Physicist]

But it only has to grab each image headers once per image, at preview time. After that, it's hardwired into the HTML. The bandwidth overhead should be negligible.

If the image server doesn't respond in a timely manner (a few seconds, I'd say), the preview screen could just time out and give a warning message. In that case, it's probably not an image that should be posted right then anyway.

[general_re]

But it only has to grab each image headers once per image, at preview time. After that, it's hardwired into the HTML. The bandwidth overhead should be negligible

If we assume that an image on some remote server - that neither you nor I nor FR has any control over - remains static, that could work. IOW, if someone substitutes a new image in under the old file name, all bets are off. Most of the time that's probably a safe assumption, but not always, I think...