This is an Ask a Geek Feminist question:
When choosing pictures for a conference website from the event held the previous year we have specifically looked for photos that happen to include women, to the extent that they are disproportionately represented compared to the actual attendee breakdown, with the specific objective of making the event seeming more diverse than it is, so that we actually get a more diverse set of attendees for the next one. This seems OK to me, would be interested in what others think.
A related point is where there is a group photo of everyone at an event, I would kind of encourage women in attendance to take part because avoiding group photos creates a negative feedback loop.
What do people think? I’m not sure. The kind of questions I’d ask before even beginning to answer this are along the lines of the following:
Is this the only/main diversity scheme for this conference? (I didn’t edit the question, so this is all the information provided.) For that matter, what do you want diversity for? For all that I and other people who write here really want diversity to be a concern for geekdom, I think having it as solely a checklist thing is a disservice to the people who will comprise the diversity. What are you offering those people? What are they offering you? Is it all one way? Is this about avoiding negative publicity or something more?
After that, are you behaving like a more diverse/diversity-friendly conference in ways other than your advertising? See Women-friendly events on the wiki for some ideas.
Finally, I am not a lawyer, but in some circumstances in some jurisdictions you may want a model release for this use of people’s images.