Reuters is the victim in this. There's no way any media agency to "check" a photojournalist's work. It's not like they can hop in a time machine to see if that's how it really looked, after all. The process depends on the PJs being honest, and knowing that faking a photo is career suicide.
Photo editors get hundreds or thousands of photos submitted to them, and they have to go through and pick out which ones to use in just a few hours. Presumably if someone did submit an obvious fake it would be caught, but the reality is that every faked photo I've seen that was used in print wasn't obvious. These two certainly aren't, especially at the low resolutions they're reproduced on web pages.
Photo editors get hundreds or thousands of photos submitted to them, and they have to go through and pick out which ones to use in just a few hours. Presumably if someone did submit an obvious fake it would be caught, but the reality is that every faked photo I've seen that was used in print wasn't obvious. These two certainly aren't, especially at the low resolutions they're reproduced on web pages.