On 4chan, it started with image macros, reaction images/gifs, demotivational posters, etc. and at some point, all of those just became what 'meme' has come to mean, and if you say the word meme without context people just assume image macros, a la meme generators.
I suppose there's nothing wrong with this, since all of those things do indeed tend to be memetic, but it made so much more sense early on than it does now.