What are the Rules of Photo Credits?

I'm not a great photographer and I certainly don't have top-of-the-line camera equipment. But I have a lot of need for quality photos at my job. That's why I love stock photography. I've been an iStockphoto customer for many years. My primary use for stock photography has been to spice up posts on my employer’s internal blog and newsletters with a crisp picture. The superior image quality and the sense of honesty I get when using a purchased photo (as opposed to the guilt felt when copying a photo from the web) are what I consider the best elements of stock photography.

It's always cool to see a photo that I've purchased used in ads for other companies. In these cases, I always catch myself saying, “I know where they got that photo!” Recently, I've had a few needs to use some of my purchased photography on external projects but my confusion about the issue of giving proper photos credits has made me hesitant. Do I credit the photographer? Or do I credit iStockphoto? Or, since I've paid for the photo, do I have to worry about photo credits at all?

I've been thinking a lot about this since I saw a recent post on MentalFloss.com that featured a stock photograph. As you can see below, the post author gave credit to ThinkStock, another online stock photography website.

It's easy to determine that ThinkStock was indeed the source of this photo. And there wasn't a watermark on this photo when it was used on MentalFloss's website - that's a good indication that it was purchased.

So I’m reaching out to you: why did MentalFloss give a photo credit in this situation? Your answer to this question will really help me. Feel free to leave your answer – brief or detailed – in the comments section below.

Seriously, any input will help. Thanks!

UPDATE || February 4, 2014: I noticed that this same image was featured in an article seen on Inc.com. The author gave credit to the stock photography provider after the article - but this time credit was given to ShutterStock!