Every single photo page can be arrived at from any other part of the internet. There's no "gateway" page they have to first enter. Thus there can [and will] be individual counts for each image, for each person who found their way there. Similarly, there will be counts for each person who arrived at your front page, with nary the two counts meeting. Each of your set pages as well, have their own unique counter.
Visits represent the number of individual sessions initiated by all the visitors to your site. If a user is inactive on your site for 30 minutes or more, any future activity will be attributed to a new session. Users that leave your site and return within 30 minutes will be counted as part of the original session.