Please see an explanation of the front page photo processes here:potd-potm-and-changes-to-photos-on-the-home-page-t64286.html
The simple explanation is that POTM shows the best photos with the highest scores by a decent handful of different members. If someone submits photos everyday, then unless a lot of other members submit photos then they have a good chance of having their photo included. Refresh the page and you will get new selections though.
If you got one photo 50% of the time then that is purely by chance. There is no actual way from someone to get their photo featured that often there, but again, how often depends also on how many other members submit images Take today for example... only 4 members submitted photos with higher scores than the Mount Charleston images, so his will be included. Their photos are included too.