[PeterisP]

Images of faces and people aren't neutral placeholders - they will affect how attractive the whole mockup (and the advertised product) will be perceived.

In almost all cases (barring special messaging/targeting), using pictures of attractive people will give better results - if you're using pictures of average people for no good reason, then you're hurting the outcome, putting out suboptimal product; it would be just like using a less-than-readable typeface for your marketing copy - unprofessional.

Gender of the pictures matters - it should be adjusted for target message and audience, but more often than not (2/3?) a picture of a woman will be more effective (attract attention and positive emotion) than a similar picture of a man. Take a look at magazine covers - both men-targeted and women-targeted magazines will prefer to have women on their covers, since that is what will get them more sales.

And that is not an arbitrary bias of publishers, this is a reflection of the true bias of the viewers, and it probably will stay that way until we're not entirely homo sapiens anymore.