“Tomorrow should have been Liz’s 18th Birthday. I wonder if she had been writing this post what she would have written. I suspect something on the lines of ‘Here’s to a lifetime of taking photos,’ ” the caption read.

Last month, Liz’s mother, Vicky Robayna, opened up to PEOPLE about their birthday wish to honor Liz, inviting people from around the world to display the teen’s images.

“We just want as many people as possible to see her work,” Vicky told PEOPLE at the time. “If anyone would like one to put up prominently, we’re up to helping with that. People can also share them on their social media.”

Liz, who had been diagnosed with a rare sarcoma, desmoplastic small round cell tumor, captured the hearts of William and Kate who heard of her desire to undertake exciting photographic assignments in the last months of her life. Hearing of her story, they invited her to take pictures of an investiture at Windsor Castle — and she and her family got to meet the royals in a surprise tea party afterward.

Back in March, Kate and William shared an inspiring tribute to Hatton on International Women’s Day to celebrate “the strength and creativity of Liz and [her mother] Vicky Hatton.”

“It was an honor meeting them both in Windsor in October,” the couple wrote in the caption.

The post also remembered Hatton for her “unwavering resilience.”