May 2 (UPI) -- Prince William and Kate Middleton are celebrating daughter Princess Charlotte 's 10th birthday. The couple marked the occasion Friday by posting a new photo of Charlotte to their shared Instagram account. "Happy 10th birthday Princess Charlotte!" they said in the caption . The image shows Charlotte smiling at the camera with a mountainous landscape behind her. Photo credit is given to Middleton, formally known as Catherine, princess of Wales. View this post on Instagram A post