I'm not sure what you're problem is, but I'll tell you what I've experienced...
- I'm always logged in, sort of. Ages ago, I ticked the "remember me" box when I logged in. In that regard, I'm here often enough and I never have to log in because I'm always "remembered".
- I've screwed around too long and I've had my post/pictures evaporate. I don't know what the issue is, but if I preview my post in a reasonable amount of time and keep the cycle alive, the forum keeps my post in memory and I carry on with more blabbering and sometimes, more pictures until I'm done.
I can only surmise, you're "timing out"... the site reloads and your post is lost in cyberspace...
.