Honestly before RES came out the proportion of images on reddit was very low. Going back further before subreddits I remember it as largely just news articles and discussions, much like HN.
Before subreddits, yes, it was mostly links to articles. But pics was one of the first subreddits, and images made an increasingly large fraction of posts over time. I think RES was as much a response to that as it was a driver of it. If anything, I think the rise of smartphones was the big push. It's simply easier to consume imagery than text on a small screen like that.
