There are more than 24,000 post offices in all over Japan. The many of them have "picture postmark". The picture postmark is one of the cancellation stamp. It draws special scenary or product arount the post office. For example, in the picture postmark of "Tokyo central postoffice" , the picture of Tokyo station, which is next to the post office.
I have collected them for totally three years. There is no limit for collecting them, but I suppoese that it is the essence of this hobby.
How to collect?
If you want to collect them, you must prepare a 62-JPY stamp and a card. Of course, the role of picture postmark is the cancellation, so you can send a postcard or a letter with a picture postmark. You can choose every card as far as the stamp is put. Then, the staff at mail counter in post office stamps on your parcel.
This hobby does not requre much money, so everyone can enjoy to collect them.