If you are referring to the poster chosen to appear on the home page of a film,
then I inform you that this choice is made based on the number of votes that the poster had.
The most voted are the chosen ones.
To vote for a poster, just go to the image page and click on the thumb symbol that appears when you hover your mouse over the poster.
It is no use voting multiple times on the same poster.
Only 1 vote per user will be counted.