In order to receive a nude photo, each donor would need to send Ms. Ward a receipt as verification. The model said that by the end of the first day — once she had added up the totals from the receipts sent to her Instagram inbox — she had encouraged the donation of about $5,000 to charities focused on Australia’s fires. She now claims to have raised more than a million dollars, though she was candid about her inability to verify individual donations. She provided screenshots in which it appeared that she had received multiple submissions per minute.

“I was trying to proof them as best I could,” Ms. Ward said. “I do think it’s very possible that a lot of them aren’t real.”

Her example has inspired others. A 26-year-old Colorado woman who poses nude using the name Lana Evans said that her husband sent her Ms. Ward’s original tweet. She quickly posted something similar and said that she stopped counting donations when she had $2,000 in receipts. She said that she joined an impromptu group of more than 20 women doing the same thing.

“I think it’s just motivating normal guys to donate because it’s giving them an incentive, you know?” she said.

These efforts, though surely more risqué, do have precedents. Partially dressed firefighters are known to pose for fund-raising calendars, and pageants with swimsuit competitions are often held for charitable purposes. Charitable nudes are certainly a more immediate means of gathering donations, although it’s possible that only a fraction of those messaging Ms. Ward and others have actually donated.