Is it legal to use Runescape screenshots and videos?

Strictly speaking, no. Here is a good article about it (related to Runescape YouTube videos, but it is the same for blogs and fan sites). No matter, what kind of disclaimer you put on your site, Jagex retains full rights to the images/videos and can (if they choose to) ask you to remove these images/videos. However, not to be discouraged, Jagex clearly stated that they don't mind fan sites to use Runescape images unless these are used in abusing, rule breaking or derogatory context. In other words Jagex retains the right to go after sites that use Runescape imagery, but don't really exercises or plan to exercise this right.