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]