try reverse image searching the picture. maybe other people are using it or something.
or, they might just be saying that because they want to talk to you and don't know what else to say. Hard to come up with something to say to girls on a dating site that would elicit a response. Antagonism, at least, has a chance of getting an annoyed response, which is still a response..
I've had someone say that to me once. Only once like, but I took it as a compliment. I just said to them, listen someone has to be that person, and I am.
And people are more aware of catfishes than ever now, so if something looks too good to be true, people are gonna start assuming it is. Or perhaps they image searched the picture, it's usually not hard to find if someone is using a fake.
Just google search a picture and see how many times it pops up. Pretty easy to tell if it's fake. So if it's fake don't complain. If it is not, then maybe put a variety of pictures up that show you in different settings/outfits.
Picture looks too groomed, there's only one picture or they all look like they're from the same time period. Best bet is to replace it with a selfie, an unposed shot taken by others (I forget the word for that), and a posed shot taken by others.