I'm very inspired by Groupon's photography from all their advertisers. They are very clean, professional and elegant. How do they create such a personal photo for a business's advertisement? Do they actually have an on-call photographer or are these mostly stock photos?
It can be stock, or they will ask the company to supply an image, it depends.
I know this because I had a photographer friend who posted a Groupon and they didn't provide him enough time to produce a sample image, so instead they just posted a stock image. He was upset by this so he asked them to replace it with one of his own.
I am sure it is on a case by case basis, as some companies might not have great imagery to provide Groupon, while others(such as photographers) obviously would.
I can offer an extension to dpollitt's answer: Groupon approached a photographer friend to be on-call to take photos for them.
in addition, groupon has an in-house studio with a studio photographer, they also book models and build a library of stock photos of their own