If you scroll down to the bottom of the ad, you will see a link to the agency that shows the number of ads they have. Or click on the advertiser number link. One I looked at recently had over 180 ads up nationally! Some claim to be independent, but aren't. And there are usually a lot of duplicate ads, that lead to the same place. Some are obvious as they have the same phone number, but some you only find out when you make contact and get given the address. As with any punt, it pays to do your research first.
As others have said, ignore the photos on the ads, they are purely to reel you in. However, some will send you photos of the actual girl or girls if you contact them via WhatsApp.
Some of these places do tend to have better girls than others. I have had a reasonably good hit rate with such ads (I would say at least 90%), but it did take awhile before I got my spidery senses attuned to which are likely to be the good and bad bets. My first rule of thumb is that the more ads an agency has up, the lower the chances of a good punt. Even so, even the best of these places will have the occasional "dud".
My impression is that in many of these places, the hourly rate that you pay is divided up between the boss and the girl, but anything you pay for extras the girl gets to keep for herself, particularly if paid after she has stashed the main payment. So paying extra for owo or dfk often gets you a more enthusiastic service.