Other mistake: In the bar scene they show an online video of Jim and Nadia. The angle of the video is from the movie but the web camera in the computer was situated in the left of the scene, so the position of the online video is not correct. (00:19:02)
Continuity mistake: When Stifler is shitting in the box, the beer near AJ falls down but is upright in the other shot.
Chosen answer: They have been friends for a long time. They had plenty of time to take pictures. We only see them on screen for a few hours of their lives spread over a short while.
Greg Dwyer