Permission from the photographer via email. The photographer releases the photo under CC-BY-SA with the following condition on its image size. This reduced image in a 600 x 800 pixel can be used for any case including commercial usages.
However, anyone who would intend to get the original image, bigger than the current image, he or she should directly contact to the photographer for the permission.  

本作品是自由作品，任何人都可以以任何目的使用。如果您想要使用该内容，只要您遵守本页面上提到的许可协议要求，您不需要请求授权。 维基媒体基金会已收到电子邮件，确认版权持有者同意根据本页面提到的条款发表本作品。该信件已被志愿者回应团队（VRT）成员审阅并存储于我们的授权档案中。受信任的志愿者可前往工单编号#2008081310016957查阅相关信件。 如果您对存档信件有疑问，请使用VRT通告板。 工单链接： https://ticket.wikimedia.org/otrs/index.pl?Action=AgentTicketZoom&TicketNumber=2008081310016957 查找来自同一工单的其他文件：