Additionally, people thought it was odd that pregnant Kate doesn't seem to have a baby bump in the pic - Kensington Palace explained it was taken "earlier this year" - and wondered if the family's all-blue ensembles might be hinting at the gender of baby No. 3.
The Duke and Duchess of Cambridge accompanied the announcement with a new family photograph.
It all started when actress and comedian Miranda Hart, who served as host for this year's charity broadcast, allowed one young guest to make a special request of the prince.
The future monarch stole the spotlight and earned big laughs - especially from his wife, Kate, Duchess of Cambridge - with a brief performance of his own.More news: Long Island Iced Tea Corp changes name to Long Blockchain
Prince George, sticking to his shirt-shorts combination, is seen donning royal blue shorts and a light blue, chequered shirt. Little Charlotte has on a blue dress, too.
Kate wears a powder blue belted skirt suit by Catherine Walker in the image.
"The image features on Their Royal Highnesses' Christmas card this year".
In 2015, the family posted an outdoors picture of themselves on the card, with blue being the colour of choice then as well.More news: Woman fined $350 for writing negative hotel stay review
But, even knowing the photo isn't new, fans are speculating it also serves as a gender reveal. "Lots of blue", one wrote.
One said: "Royal baby definitely a boy!"
Every year, Britain's royal family picks out a "postcard perfect" picture that will go on their Christmas Card.More news: Pentagon had multi-million dollar secret UFO programme
Yesterday, odds were slashed on the abby being named Alice or Arthur after bookies received a flurry of bets.