Does having images help your ranking?
If I were to define "good thick content" in a way that a robot could understand, I might look at the following variables:
- High wordcount, let's say maybe 1,000 words or more
- At least 1 relevant image on the page
- Bonus points if it's an original image (i.e. not a stock photo)
- High ratio of words to links out
- Lots of social shares
Going with that profile, it's possible that ask.com etc. were the intended targets and Metafilter was collateral damage.
This is all speculation so take it with a grain of salt.
How would you figure out relevance algorithmically?
Reverse image search is probably too expensive to do this for every image found on the web, but Google could surely do this for a subset. Then they could match alt attributes: If two sites use the same image, and one has an alt attribute of "automobile" and the other "car in street" you can ask Bayes how likely it is that these two sites both made fake alt text that does not describe the image.
Basically if Google Images can show you images relevant to your search term, then Google Search can find out if an image is relevant to a topic in much the same way.
Directly: It adds rich content to your posts (video's, audio, images). It adds keywords in the image alt attribute and image description. Good images will increase your ranking on Google Images.
Indirectly: If the images contribute to the topic/post (not merely decorative), then you get more user satisfaction, which will increase repeat visits, word-of-mouth and links.
Do the all-things-equal test: Two pages on the same topic (let's say 'Bonobo Monkey'), but one has a picture of the topic. Which one would you find more useful? Two pages of equal quality, yet one has a picture of the author with rich markup next to it, which one would you find more credible?
As with all things, don't overdo it, or use it to manipulate: Adding images when they don't make sense for the topic, copying all your images from elsewhere, spamming the alt attribute with keywords, put a page full with unrelated images etc. Basically do not treat it as an SEO technique, treat it as enhancing your content with media to increase overall quality and user satisfaction.