Yes, you must login first on many or all devices because image button uses a "folder" under your user name on PFF server.
If you have text already and don't wait to lose that but you haven't login... post the text w/ login info, that logs you in, then Edit the post or add a post to add picture(s) to a thread.
Don't trust you login in, then close the tab/browser, then open browser or new tab again and try to upload pic in first post of the new session.
Many browsers doesn't play well w/ many cookies, scripts and other methods of "auto login" at many sites not just PFF.
"Auto Login" is Not = to Stored User Names and Passwords to fill any form like PFF Reply To posts.
Many sites has a box to remember you, browser and more to auto log into the site... Browsers may fight this to start w/. Add NoScript and other plugin's then they often won't allow that function.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave