The tweet has been shared more than 1300 times in 17 hours.
UNICEF has condemned Williams' use of the images, which were not taken on his trip to the war-torn country.
Spokesman Patrick Rose said UNICEF was an organisation dedicated to protecting the rights of children throughout the world.
"We see it as a fundamental infringement of those children's rights. But at the same time, we don't have the capacity to sensor or edit private citizens' showing what they find on their individual explorations online."
Williams is not an official ambassador for the charity. Mr Rose said the rugby star's trip was to help him understand the crisis and to help raise awareness for the refugees' plight.
Mr Rose said it was obvious Williams had been "deeply moved" by his experience in Lebanon but didn't agree that the photos were the best way to help refugees.
"We [UNICEF] are as disturbed as anyone when we see those images but we want to offer a positive framework for people to respond to that.
"We can't stop the war, we can't stop these things happening to people, but what we can do is help children by getting them clean water, by getting them counselling to help them deal with that trauma and to help them have a better future by keeping them in school."
UNICEF had tried to contact the rugby player about the photos, but had not yet been able to reach him and voice their concerns.
"I don't think anyone would be happy about those kinds of images. It certainly wasn't something that he consulted us about and they weren't images that he'd taken on the trip with us."
The public response to the photos has been mixed.
Some have praised Williams for bringing attention to the crisis, and others say it could incite dangerous reactions and was disrespectful to the dead.
Williams' All Black and Chiefs teammate Liam Messam tweeted: "Good work, my brother."
Lebanon shelters more than 1.2 million refugees from the Syrian conflict, which began five years ago.
