Zoia, On the main page for "Site Banners" click on "Upload a banner." It will show a list of your banners that you have submitted, and it will indicated if they have been "approved" or "waiting for approval" or "approved for yyyy/mm/dd." (i.e. the date it will be used as a banner). The new banners are posted the evening before that date.
If a banner you submit is not approved for some reason, it just disappears. I've had 1 or 2 rejected---not sure why.
Poor resolution, or out of focus pictures probably are the main reason for rejection. Or something that just doesn't represent gardening. Maybe pictures with people or hands might not make it----I don't think I've ever seen a banner with people in it.