Reproducing figures in blog/SE 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”.