The pictures are being received and stored on the disk, as you watch you are working your way through them. When you pause you are simply suspending the watching process but the pictures are still comming in and being stored. When you start again you start from where you where last looking, imagine it like a book mark, except the book is adding pages to the back and removing them from the front as you read so if you stop reading the pages just get added to the back for you to catch up with later.