Welcome to this forum Dennis, and we hope to see many more of your interesting images. To answer your specific question, you can post only one photo per week in the weekly theme thread, and one photo per day in the 'photo-a-day' and the 'image art' threads. You should also familiarise yourself with the photo posting guidelines set out here
and the people image posting guidelines here
Old photos can be posted, provided they were taken by yourself, but the main idea, especially in photo-a-day is - I think - to show fairly recent images of sights you've seen.