There is a fullscreen icon. When you click it, it changes to the inverse of fullscreen (lack of a better word - icon for “smaller screen”). This is how it works here (PlexWeb Hosted)
Oh. I may have misread the OP’s question. Are you looking for the button to close the web browser (there isn’t one in full screen mode) or the stop button to stop showing photos and get back to the menu (there is a X at the bottom for this)?
I may have to claim stupidity here but this morning I opened up a picture in a plex library and it was not minimized I had no control bar at the bottom and no way to close it without minimizing it. I repeated enough times to know that I am not crazy but today when I got home I cannot reproduce the problem. Now I do have a bar at the bottom that includes an X to close. At this point I can only assume this was a problem with my web browser and I assumed it was a side effect of an update without “rebooting” and testing again. Sorry for any confusion. If I can make it happen again I will repost.