This picture really has nothing to do with tire width, except that your boy was using 23s when this was taken. Mt. Lemmon, January 2014. Photo credit Brendan Halpin. www.thecyclinghouse.com
One thing is for sure - we absolutely recommend using tires when you're riding on your Rails. Failure to do so will, in fact, void the warranty.
Many people ask us if they can "still" use 25s with Rails. One of those questions that we get often enough, but which I totally don't get the genesis of, so I assume I'm just missing something, or something we've said is super easy to misconstrue. You can use WHATEVER tire width you'd like on Rails, so long as it is 23mm or over. I've got a set of 33mm Grifo CX tires on a pair of Rails, and soon enough I will have a set of 2.2" Maxxis Ikon 29er tires on Rails.
As far as what we recommend, we really don't. It's all subject to personal preference and intended use. For racing, I've found I like 23s. Tire pressure at about 75psi works great (yes, I did pinch flat once last year, in a huge pothole at Green Mountain Stage Race - the guy behind me pinch flatted a tubeless tire in the same pothole, nothing was getting past that sucker), and I've yet to measure a stated 23mm tire that didn't measure much wider than advertised on Rails. Continental 4000s usually go north of 25mm wide even when new. Absolute measurements of tire widths really do seem meaningless, except that when I've tried "25mm" tires on Rails, they're HUGE and have seemed to slow steering down past how I prefer it.
We also don't recommend specific tires, just because there are so many good tires out there in the world, and they all work great with Rails.