A few answers, as short as the rule itself.
How often can I post?
Once a day. That’s the whole point.
Who sees my stills?
You decide in Settings: only you, the people you follow who follow you back, or every member of OneStill.
What is a still of the week?
When a week, a month or a year is over, you may choose one photo to stand for it. The choice is final.