Reproducing figures in blog/SE posts Sometimes when I am writing a review blog post (or an answer on SE), it is convenient to include a figure from the original paper. Some journals (say PNAS) have policies that explicitly allow non-commercial reproduction of figures:


  Anyone may, without requesting permission, use original figures or tables published in PNAS for noncommercial and educational use (i.e., in a review article, in a book that is not for sale) provided that the original source and the applicable copyright notice are cited.


Others, however, seem to explicitly disallow this, of note is the Nature Publishing Group. Today, I had to fill in an online form on RightsLink in order to ensure I could use a figure in my post. I didn't have to pay anything, and the rights were granted instantly after completing the form, but it was still a hassle. The biggest hassle is having to go to each journal's website to check their policies. Hence the questions: 


Is there a general law like fair-use that allows me to place figures from published papers inside blog posts for non-commercial commentary/review purposes? 
What if your blog has ads that generate revenue, is the use of the figures no longer non-commercial? What about SE that generates revenue but not for the poster?
Is there any extra etiquette one should keep in mind for including figures in blog posts?
Some thoughts on this issue:


First, the fair use doctrine is tricky. Assuming you're not ready to hire a lawyer, play it safe. If you're not sure, ask for permission. If you're asked to remove something, do it.
There has been some noise a few years back on the topic of blogging and reuse of scientific figures. See here, there and there for some links to that affair. The conclusion I would draw is, again: play it safe.
In at least some jurisdictions (France is one), having ads on your web will mean your blog is considered a commercial publication. Asserting fair use for commercial works is typically harder, though this distinction tends to diminish (here).
Attribute figures (source + link). Always. It's just good manners.


I'll finish by a general observation related to copyright law in academia, as I see it (at least around me): people tend to just do stuff, and then play dumb if they get caught (which rarely happens). Lots of researchers knowingly put up (without authorization) PDF files of their papers on their website, and say “I'll just remove them if I am asked too”.