Generally speaking, any use of copyrighted/trademarked material requires licensing.
The site you linked, if it isn't licensed by nintendo or the pokemon company, is operating illegally, and could to told by nintendo to shut down. Since ign is a reasonably big gaming news/review site, I would think that they have licenses with various publishers to use their content for certain things, such as making game guides.
However it is important to note that using such material may fall under fair use if used for purposes such as reviewing, which is what a lot of gaming sites do (though in this case this is a guide by ign and not a review)
That said, there are many small sites which infringe on nintendo's copyrights, but because of the cumbersomeness of sending cease and desist letters and the potential costs of legal action, only the most financially problematic sites are taken down. (romulation.net, for example)