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.
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...