It's a good strategy for them to update their pictures frequently. Whenever I see an ad with "New Pics" in the title I always click on it.
The professional Glamour shots are always nice but I prefer the "selfie" or amateur like pictures, I know those can be retouched as well but it gives the feeling that those shots are more 'real' for a lack of a better word.
The new pictures keep it interesting and fresh, even when it comes to my favourites that I repeat with often, it's nice to see new pics and a few times it's influenced me to book on a weekend I wasn't intending to book or changed my mind on booking another SP just because the picture I saw was so appealing that it activated my mojo. The pictures don't even have to be them half naked in Lingerie either, could be a sexy picture in a nice dress or even casual clothes.
On the flip side of that there have been times where I was reluctant to book just because the pictures didn't appeal to me or weren't flattering, but just based on the reviews or reading about them on their websites or ads appealed to me enough that I would book a meeting, and 100% of the time so far the pictures did not do them justice, they looked a lot better in person.
Some women have a good eye at knowing which pictures they should put up and some need some help selecting the most appealing ones.