In the very very opening scene, when it's scanning the Doc's lab, there are tons of clocks. One of the clocks has the Doc hanging off the edge of it, just like in the end of the movie!
Doc already lived through this whole experience by the time the movie started. He probably never told Marty because of his feelings of the time paradox, etc, etc. So Doc easily couldn't have recreated his experience of hanging onto the clock to get a picture of it.
You're wrong. It's really a picture of Harold Lloyd from the silent movie 'Safety Last'. He climbs up the side of a building and winds up hanging on to the minute hand of the big clock, just like Doc Brown did. It's more like a background joke.
Also just to add to the above comment, none of it has happened yet because the Doc just invented the time machine and Marty hasn't even traveled yet, which means it hasn't happened yet.
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