So, back to the original question, "Can photos be trusted?". No!
In the modern era, photos are like any other form of information -- they can be trusted only if their source is known and can be trusted. It is now as easy to manipulate images as it is text, so the same basic rules apply to both.
On the Reuters "bathroom" photo, there's no credible evidence that it's a fake. Reuters admits to cropping it and enhancing the contrast, both of which are standard practice for news photos. To believe that it's completely fabricated, you'd have to believe that Reuters would be willing to chuck out its credibility and burn a photographer with a 30-year professional career, the last 12 years as a White House photog with a good relationship with and good access to the White House.
More importantly, what does the photo show? President Bush was in the UN Security Council, not somewhere he spends a lot of time. He asked his Secretary of State, the top diplomat in the country and someone who might be expected to know protocol, about whether it was appropriate for him to get up and leave the room while the session was in progress. It's a reasonable question to ask; you don't want to risk a move that another leader will interpret as a snub.