It was a Reddit user that copies a link and sends it to Imgflip because Reddit is a source for users that can post a photo on itself and on Imgflip when it's used by a Redditor from Reddit.
I'm sure it was shared by a redditor. I have heard that someone said that once someone is anonymous and is shared by a redditor it says "made by a redditor" since you can share imgflip images on Reddit.