They're the sample pictures that come with Windows Phone 8. They're also the images that come with the emulator.
You've no doubt seen them loads of times, probably when working with images on the emulator.
Using them while testing is perfectly fine.
However, there is no excuse or reason for ever using these images within your app when you're taking screenshots for use in the store.
Yes, if you're taking screenshots via the emulator using the images that are on the emulator is easier than adding your own, but the store is about selling. Doing what's easiest for you is hardly ever the best approach to take in the long run. It also doesn't do a good job of selling your app.*
Also, using these images in a context that is completely irrelevant (for instance, I've seen an app that relates to faces including the above footballer picture, and an app that should have had an image relating to a couple but instead showed a pair of legs next to a swimming pool.)
Using these images in your screenshot sends out the following messages:
- You're lazy - if you cut corners here, where else have you cut corners? In testing? Is the app full of bugs?
- You don't care about how your app is perceived in the store - Oh, you don't care about how it looks in the store. What else don't you care about? Providing support?
- You're not interested in making your app understandable by viewing the screenshots in the store - How about making it easy to understand when trying to use it for the first time? Or subsequent times?
*But, what if you claim to just be a developer, not a salesperson? Well, you're actually performing many roles when you build and release an app. You can't hide behind the excuse of "just being a developer" if you want to be successful. You have to do many tasks and perform many roles when releasing software. Writing code is often the simplest of them.