I am assuming that the image resizing part is why it takes so long. Unless the pictures are absolutely massive, could you do it in a way where you just set the width and height of the images instead of actually resizing them. It may not look amazing in a browser, but if you output that to a PDF...
a session isnt a cookie.. a session id is stored in a cookie, but that is just an encrypted string.
i use sessions all the time in php here at jodo and i have never had any problem with them not lasting.
there is really no other logical way to do a login system. if you are worried about...