If we are talking about individual surfers looking at something at the same twice I would say that often (though not always) they would look at it at the same time of day (depends on their schedule though).

I would however make the point that no matter what time you change the image every day, international surfers throw another whole item in to the equation. If you are changing it at 5pm everyday then this could mean 2am for someone else (not that this would really matter).

Allan