Natasha Heidlage took the photo during a break in Scotland with pilot Matt Smeller, 47.
He died three months later from a rare virus after they returned home to Dallas, Texas
Natasha, 42, dedicated her £1,000 win in the annual William Hill Award for the ‘best Nessie sighting to Matt.
She said: “This contest has meant so much to me. Matt was so excited by the picture.
“He knew it was unusual and thought it was cool that we might have captured the Loch Ness Monster.
Ms Heidlage said the sighting was very brief but she was adamant something was under the water.
She said: “Initially, I thought this was a shadow on the loch, however, there is no possible way it could have been.
“It was very cloudy, there was no light refraction taking place at all, nor was anything tall behind me that could have made that reflection.
“We were out toward the middle of the loch and the shadow was large, with a long slim part extending out to a large area.”
Gary Campbell, president of the official Loch Ness Monster Fan Club, described the photograph as very interesting, particularly as it is taken on the loch as opposed to from the shore.
He said: “This is actually quite an unusual sighting. At first glance you think it’s potentially a shadow from the boat she was on, but she provided comparative photographs from either side and explained why it couldn’t be.
“It’s really interesting – it could be that the Loch Ness Monster is actually coming up to look at boats and potentially following them about.”
He added he had been impressed by the quality and variety of this year’s entries, which were all reported by people living outside the area.
They included photographs taken on the loch and from either side of the loch.
“It shows what a global phenomenon Nessie truly is,” he said.
The 2014 prize went to Bjarne Sjöstrand, 52, for a picture taken from his home in the Swedish capital, Stockholm, via Google Earth.
Natasha added: “Matt told me, ‘You have got something here’, and we showed the picture to the couple who ran the B&B and they said, ‘You need to submit this to the contest’.
“We had another picture with something like a long neck sticking out the water, but I didn’t send that in.
“Matt liked Scotch and we’d gone to visit the distilleries but Loch Ness was my idea. I was just snapping photos. I didn’t see anything with my naked eye.”
She added: “Later in the day I was going through the pictures and saw the dark image in the water.
“I was startled because it was overcast and there was nothing that could have been behind me to project a shadow.”
“We got engaged after the Scottish trip and were going to get married this year.
“We had been looking for a meteor shower when he was bitten.
“The next day he got the flu. The neurological symptoms came five days after, the double vision and extreme dizziness.”
Matt later on died from complications in September.
Natasha, who took her photo on 1 May last year, said: “I am an avid traveller and intend to return to Loch Ness with a friend.
“I’d like to find the Yeti next – there is more exploring on the horizon.”
William Hill spokesman Rupert Adams said: “Hopefully Natasha will use her winnings to solve great American mysteries such as Big Foot or perhaps even a trip to Area 51.”