UNLIMITED changes---you can change your pictures every day if you want to. (Note that site improvements, adding new pages and section, or new services are not update/maintenance and are not included in the update/maintenance agreement.)
Six months of update/maintenace is $72, which works out to $12 a month.
The update/maintenance service is also available in a one-year package, with one month free, so a year agreement is only $132.
Update/maintenance service is paid for in advance, and then you get six months (or a year) of update/maintenance.
