The bottom line is this:
1. Elise Andrew is making money off IFLS. (Whether or not she's giving it out to charities is non-relevant.)
2. IFLS' success rests largely on the high quality content created by artists, scientists, photographers
3. A small part of IFLS' success rests on Andrews' eye for picking out good/funny/clever quotes/cartoons/photographs/artwork
Given (1) and (2), I'd say not only is IFLS largely indebted to the artist/photographer/science community at large, but not attributing work is an incredibly selfish thing to do. Whether she didn't attribute the work out of laziness, negligence or malice is non-relevant. My only point is that IFLS is a very good target for the blog post.
If someone stole my code and used it without attribution and to make money off of it, I would be pissed. If someone that's received the major media attention IFLS has stole my code and used it without attribution to make money, I'd be even more pissed.
Also note that artwork can have varying licenses, as well - similar to MIT, GPL, etc. licenses we see in source code. Some licenses preclude all non-endorsed distribution. She's had a ton of media coverage in the past 6 months (especially after the whole Twitter thing), so the "small corner of the internet" argument won't fly.
Neither will the "Reddit/9gag/etc. does it" argument. Reddit, for example, is a mass-user-driven aggregate. Reddit can't possibly police all posts and ensure that attribution is given. Conversely, the IFLS page is not an aggregate. Andrews picks the images very carefully and in a conscious manner; in the latter case, there is clearly room for attribution.
However, I think that IFLS has garnered the same type of militant defenders as /r/atheism/ or fans of Carl Sagan (or, more recently, NDGT). Ironically, this type of blind following is exactly what the aforementioned groups are trying to dissolve. Andrews screwed up. Badly. This is not a rookie mistake; it's something she could get sued for. Lets leave it at that.. enough with the apologists.
Edit: I would just like to re-iterate that the burden does NOT lie on the artists to scour the internet looking for mis-attributed pieces of artwork. But rather, on the distributor of the work. Just about every judge in existence would agree with this.