The 5 images displayed are the 5 photos most recently added to your Wall Photos album (aka posted to your wall) by an admin of your page.
Images in the photo gallery strip display in random order.
Despite contrary reports, it appears that images posted to your wall by non-admins do not populate the photo gallery strip.

Traditional news outlets have some tough decisions ahead if they believe programming is part of journalism’s future. They need to decide where to place the people who do this kind of work, whether it’s in IT, the department that manages the website (which may be separate from IT), or in the newsroom. They need to figure out how they’ll balance the day-to-day work of running and improving the website against news-related programming. They need to find and retain employees who express an interest in and talent for this kind of work.

“One of the things we have once a month is InDay. It’s on a Friday and there are no meetings allowed; it’s a day where you get to do whatever you want. We bring speakers in, people arrange classes, but you can also just decide that you want to work on a project and you plan it out. And you may get 3 or 4 people that say, ‘That’s a really cool idea; I want to help you with that.’”
 
“So you get a bunch of guys, and you take a chance on it. And maybe it takes a couple of InDays to get it over the line. That’s how a lot of our innovation happens. A lot of great stuff is created by having time and flexibility to foster the creativity, come together to find solutions, and to make them real.”