The original meaning:
https://en.m.wikipedia.org/wiki/Meme
The word meme was “coined by British evolutionary biologist Richard Dawkins in The Selfish Gene (1976)[11][16] as a concept for discussion of evolutionary principles in explaining the spread of ideas and cultural phenomena. Examples of memes given in the book included melodies, catchphrases, fashion, and the technology of building arches.[17]”
[1] https://semioticon.com/virtuals/imitation/van_driem_paper.pd...
[2] https://vtechworks.lib.vt.edu/bitstream/handle/10919/42774/b...
[3] https://en.wikipedia.org/wiki/The_Leiden_school
Edit: Formatting and typo
I was aging out of SA before 4chan even really got going (not maturing out, since I'm not that mature; just being done with college, moving on to professional school; not spending as much time on forums/etc). I never spent any real time looking at 4chan.
I of course know what image memes are, but if you just said meme to me with no context then I still think of it as the Dawkins 'Selfish Gene' idea.
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.