The word "meme" was coined by Richard Dawkins in The Selfish Gene to allude to the concept of a unit of cultural information subject to mutation and selective pressure. The whole image-macro-overlaid-with-text interpretation is just an easily evolved exemplar which rose in popularity relatively recently.

A screenshot of a comment can effectively communicate a cultural concept. In some cases, much of the context of the concept being communicated is intrinsic to being a comment (green texts, Tumblr comment chains, etc).