I left a broader (and a bit rambling) response to some of the general points here to facetofcathy below, but I did want to make a note about the fanfic/fanart comparison. One issue is that it usually is possible to talk about fanfic pretty clearly just by describing and quoting it - whereas "quoting" art is almost always going to involve showing it, just because the way we understand visual images is different than the way we understand text. If we're talking about preserving the meaning behind a discussion of a work, then if a piece of fanfic disappears from where it is linked to, then the quoting/discussion will probably still make sense, whereas with art it is much less likely that a description can do justice to the commentary. Unfortunately it's just a difference in the medium. As far as how much of the art is appropriate in order to maintain that meaning, that's why we consider issues like resolution, size, etc.