Wikipedia:News agencies and fair use
Wikipedia's non-free content criteria requires that fair use images "not [be] used in a manner that is likely to replace the original market role of the original copyrighted media." We have all used the "low resolution", Wikipedia is a non-profit, and other excuses that it won't affect commercial use before, but there are cases where a specific class of photos cannot be used for illustrative purposes on Wikipedia, even under fair use.
News agencies often supply news, video, and photography to news outlets for use in their works. In most cases, they don't get this content for free, these outlets have to license their content from them for fees, which is why you usually see copyright disclaimers often on Associated Press reports syndicated onto other sites. Of course, we all know that the AP has been rather strict in this field, trying to even assert property rights over the concept of breaking news.
How this conflicts with NFCC 2
Unfortunately, since these images must be licensed if they are to be used to illustrate their subject, using them on Wikipedia, no matter how low-resolution they are, to illustrate the subject of the article, is us using images for free that others have to pay for. Since this is the original market role, in most cases, press agency photos cannot be used on Wikipedia, when they are used only to illustrate the topic of an article.
However, there are cases when a press agency photo may be used. They can be used on Wikipedia, if the photo itself is the subject of the article. You would expect to see [[Raising the Flag on Iwo Jima on our article for Raising the Flag on Iwo Jima. Since this article is talking about the historical impact and effects of the image itself, it is a transformative use, allowed under our policies.