The image of her and boyfriend Jeremy Parisi was taken at Royal Ascot on Saturday.
In the original version, the 38-year-old model and TV star has more of a rounded stomach than in the one she altered.
Defending her decision to change the image, Brook said she had been in a car for two hours before arriving at the Berkshire racecourse, and her expensive dress was “really creased and a little bit tight round the top part”.
She added: “So I just thought I would smooth it out a little bit.
“There are apps on my phone and I can just tweak it and make it look better.
“I got a bit carried away and I took my tummy in a little bit – I did do that.
“It’s not like I’m really drastically changing how I look.”
Brook said that after putting on weight as she got older, she was “embracing being a bigger size”.
“I’m not on a diet, I’m not telling other people to lose weight and this is the size you want to be,” she said.
“I was just trying to make the lines of the dress look a bit nicer – I really don’t overthink it.”
Appearing on ITV’s Loose Women, she pointed out that her appearance in the TV studio was not “real”.
“I’ve just spent two hours in hair and make up,” she said.
“I didn’t wake up like this so nothing is real. How far do you want to go?”
Being in the public eye was partly why she changed the photo, Brook said.
“I just think because I’ve been in this industry for so many years I am always looking at pictures in a different way,” she added.
“I like creating beautiful images and lovely pictures. Even if I take a picture of my garden or if I’m on holiday and I take a picture of a beach I always put a filter or something on it to bring out the sunset.
“So when I look at myself I think: ‘Oh, that’s a nice picture but I think I can make it a bit better.'”