It appears to be a mix, depending on the business and the deadline. Based on the community answers, these ads may use:

• photos supplied by the business
• stock images when a business doesn’t have usable images ready
• photographers hired or kept on-call to shoot for the platform
• in-house studio production, including models and a proprietary stock library

So there usually isn’t one single method. A company like Groupon may use whatever is most practical for a given advertiser while keeping a consistent visual style through art direction, lighting, retouching, and layout. If a business has strong images of its own, those may be used; if not, stock or newly created images can fill the gap.