I haven't heard anyone saying people could be looking for images on sites like beautifulpeople.com, instead of Google search. Is there an increased risk of being caught than there is on Google Images?
Some dating sites actually explicitly let you use images from their users on your landers / ads, which is great from a legal point of view. They're mostly the Dream Marriage / Anastasia Date sub-category of dating, but it's always worth asking.
I wouldn't recommend grabbing images from the sites you're pushing without permission. That's likely to upset people.
If the advertiser provides you with it, then you're good.
Using them directly from users on the website is probably not allowed. You likely won't get in trouble though, but it's happened before and it's an ethical question as well.
Just don't be the idiot who uses the picture of a dead girl and you will be fine....