I am just talking about my own experience. But there are thousands of papers about early language development (which I have not read).

You could start with a google scholar search like https://scholar.google.com/scholar?q=storybook+OR+%22picture...

OK.

Was wondering if that was research based or just personal experience. The hypothesis makes sense, but I'd love to see it substantiated.

I’ve been looking into the research on second language acquisition a bit, and the primary mechanism for learning a language appears to be hearing/reading understandable sentences for content, rather than structure. When learning, some sentences in the text will make sense to you and others will be out of your reach.

I suspect that the role of pictures (or any other context) is to help correct your mental state of the narrative world so that the sentences you don’t understand don’t present an impassable barrier, preventing you from getting to the later sentences that will help you learn.