Below is a link to a pdf of the formal rules of competitions.

In summary:

• There are usually two open, one theme and one panel competitions per year, though this may vary.
• You can enter up to two each of prints and digital images per single-image competition and three to six for the panel competition.
• Hand in prints to the competition secretary at the hand-in date, which is usually the meeting before the competition (typically two weeks before).
• Digital images should be no more than 1400 pixels wide and 1024 pixels high. Here's a pdf with instructions on resizing.
• Put digital images and their titles in Photo Entry.
• Put print titles and digital copy in Photo Entry.
• High-scoring images will be kept for use in external competitions.