According to The Anne Frank Center, USA, Otto Frank (Anne's father) loved taking photographs.

"After the arrest of the eight people in hiding, Miep, Bep, Jan Gies and Van Maaren, the warehouseman, entered the Secret Annex. It was in chaos. The pages of Anne's diary lay scattered on the floor and were gathered up along with other papers and books and taken downstairs. The Frank family's photo albums were also retrieved. Otto Frank loved taking pictures and this is why there are so many pictures of the family. About a week later the whole Secret Annex was emptied on German orders."

Source: http://www.saatchigallery.com/museums/full-museum-details/faqs/ac_id/94

Photography was a popular hobby among many middle-class Germans in the interwar period. The Franks were assimilated middle-class Germans who emigrated to the Netherlands after Hitler assumed power in 1933 and still retained some wealth and property. Her father Otto created his own business and the Frank family, while not rich, had some means.

The persecution of the Franks began with the German occupation of the Netherlands in 1940 and the occupation instituted various restrictions against Jews and began clamping down on Jewish movement and expropriating Jewish property. However, the Germans instituted these restrictions in a peace-meal fashion between 1940-42, so there was time for Otto to take photographs and have them developed before the family needed to go into hiding.