Anyone can explain me why they made a Photo book and even on the beach, but without Bikini pictures?
Is bikini in Korea maybe forbidden like showing private parts in Japanese "movies" (mosaic)
Modesty-reasons. Although SNSD and most girl groups have suggestive outfits(see Brown Eyed Girls' "Abracadabra"), they normally do not wear swimsuits. Even in the Cabi MV, they opted for a tank-top and shorts. During Ga-in and Jokwon's Bali vacation, the producers and the management company told her that she was not to be shown in a bikini.
The closest I've seen any member of SNSD in a bikini is during this collection of Tiffany's eye smiles(which by the way...
). At around :35, you can see she's wearing a swimsuit top.