The photo offered a disconcerting tableau that at first glance was hard to believe: It appeared to show a room inside the center, with an old popcorn machine, a black-and-white cat, a lamp and a table with bottles of water. Several residents appeared calm in their wheelchairs and armchairs.

But all of them were waist-high in murky water.

A Twitter user, Timothy J. McIntosh, posted the image on Sunday morning. Other Twitter users expressed shock, and some said the image was fake.

Mr. McIntosh insisted it was real.

He wrote on Twitter that he was in Florida and that the image involved his family. He did not respond to requests for comment on Sunday, but he told The Daily News, a Galveston County newspaper, that his wife’s mother owned La Vita Bella and had taken the picture.

The image gained widespread attention on social media, where thousands of people shared it, and many sought help from emergency responders and local news outlets.

Ms. Bobrick first saw it in a text message from her daughter. There in the background, she recognized her sister, Ruth, whose family declined to share her last name to protect her privacy.

Ms. Bobrick said Ruth, 64, had a blanket draped over her shoulder. “That is my sister,” she said. “And she has a long-term head injury from a drunk driver who hit her when she was 19 years old.”

Ms. Bobrick said she had no doubts about the photo’s authenticity; she recognized the center and the level of flooding matched what she had already learned.

“They did look sort of calm in that photo,” she said. “But at this point in time they had been in that water for several hours. When you get people that are in wheelchairs, or cannot get around, or they have walkers, if the water kept coming up, what were they going to do?”

On Sunday afternoon, Ms. Bobrick said, a center employee called with good news: An evacuation was underway and the residents — more than a dozen — were being relocated.

Ken Clark, a Galveston County commissioner, confirmed on Sunday that the residents had been rescued, though he could not say for sure how many.

He said the furor over the photo was not what brought emergency responders to the scene.

“We knew about it before it hit social media,” he said. “We were working on a solution for the nursing home, and it was in progress, so social media can sometimes leave one with the wrong impression.”

Ms. Bobrick said she was glad that the photo was posted online because it might have prompted a quicker response. She added that she was saddened by the other desperate situations she had seen on social media — photographs of people trapped on roofs or struggling with small children.

“It’s tough here right now,” she said.