(CNN) A British girl seen by thousands in a heartbreaking photograph that her father said showed "the darkness that is childhood cancer" has died, he announced in a Facebook post. Jessica Whelan, of Clayton-le-Moors, England, was 4.

"I feel both sadness and relief in informing you all that Jessica finally found peace at 7 o'clock this morning," wrote her father, Andy Whelan. "No longer does she suffer, no longer does she feel the pain of the physical constraints of her body. Now my princess has grown her angel wings and has gone up to play with her friends and loved ones. She will now watch down over her little brother and ourselves until one day we are reunited again."

More than 100,000 people had signed up to get updates of Jessica's battle with neuroblastoma after she was diagnosed in September 2015, and they followed along as she underwent treatment for the disease. But it was a black and white photo her father posted in October that seemed to affect people the most.

In it, Jessica can be seen with tears rolling down her cheeks, her eyes closed as her body tenses upward in pain.

"With this photo I do not mean to offend or upset. I do mean however to educate and shock those that see it in it's context," he wrote. "Perhaps by seeing this photo people not in our position will be made aware of the darkness that is childhood cancer, perhaps these same people may be able to do something about it so that in the future no child has to suffer this pain, so that no parent has to bear witness to their own flesh and blood deteriorating daily."

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