I think a "repost" is a meme that you find anywhere on the internet and then repost without altering whatsoever.
Example for comments: If I react to something someone says with the well-known "no no he's got a point" from The Emperor's New Groove without adding any additional text or captions myself, then I would say that is technically a repost of a meme, though most might not necessarily think of it that way since it's not being submitted as a standalone image to a stream.
I do like to repost quite frequently, myself. I follow several Facebook groups which post memes and images on topics I find interesting, and I sometimes repost their content. Sometimes I just like to repost funny things people have said on Twitter or cool images on Instagram.
I think it's perfectly fine to repost, as long as certain guidelines are followed. I always submit my reposted memes in the repost stream, conspicuously title them as "repost," or at the very least slap the "repost" and/or "reposts" meme tags on them.
As for original images by memers, this was a cool stream I found the other day where memers submit their own original photos.
imgflip.com/m/ShareYourOwnPhotos