Another graph form Institute for pointless 4chan studies.
This chart represents ratio between post containing some variation of phrase pic related and pic unrelated. Related part is under each line, unrelated is above it. You can see that in most cases pictures are related in about 80 to 90 percents. But there are some less related boards: /r9k/, /g/, /x/, /adv/ (you can see declining trend here) and /r/. Request board contains about the same number of pic related and pic unrelated images.
Chart is again heavily smoother (each point is averaged with previous and next days). Missing data in my dataset (they are nicely visible in my last graph as huge drops) was masked by simply using last valid values instead missing ones. Nevertheless you can still see wild fluctuation around November 2011 and May 2012 caused by lack of data.
As a bonus there's table sorted by how many pic (un)related posts are OP's posts.
Raw data here.