"feeds of" is not a set pairing, which may have led to your confusion. Feed in this sentence is a modern day noun usage which refers to the list of pictures or statuses on the front page of a social media site, such as Facebook or Instagram. Of in this sentence is just the preposition to connect to the subject of pictures to the word feed, which is the list that I mentioned above.

Here, “of” is a preposition that introduces the adjective phrase “pictures posted by people they follow.” That adjective phrase describes what is in the “feeds.” You could rewrite as “feeds that consist of pictures posted . . . “

This is not an idiom. "Feeds of pictures" refers to the content[pictures] of what people have posted on either instagram/ facebook, or other social media. "Feeds/feed" in this sentence is commonly used by/in social media now a days in reference to posts or ongoing posts online. Facebook refers to it as "news feeds", Instagram refers to it as "Instagram feed" [series of pictures that you post in your Instagram account. Pinterest refers to it as "Pinterest feed", etc other example use of news feed/feeds: "How to Visually Plan Your Instagram Feed" [title of a blog] "News Feed is a feature of the social network Facebook" "The Facebook News Feed is made up of stories from friends, followed Pages, and groups joined."[from facebook]

Here the word "feed" is used to talk about the content you see on social media pages. For example, if I'm on facebook, I scroll down the page and all the different posts from people I see are my "feed". For Instagram, I scroll down the page and see pictures from the different people I am following. This is my "Instagram feed", it's basically just the content I see from everyone on my social media page.