So it turns out that a lot of people are getting reblogging and reposting confused, which is possibly one of the reasons that reblogs - especially of things like artwork - are so significantly down. I personally have always thought the two terms were too similar and bound to lead to confusion; idk why "reposting" was the term that was chosen, but whatever.
Re-BLOGGING means sharing a post. It is taking the post that you can see on your screen on tumblr, and sharing it to your own blog on tumblr, meaning that anyone who follows you will see it as well. The reblog automatically includes a direct link to the blog of whoever made it. Creditting the original poster is inherently done when reblogging, because of the way this site is structured. Reblogging is a GOOD thing. Reblogs are great. Authors and artists and gifmakers and etc, we all want our posts to be reblogged. Reblogs are also tumblr-specific. You cant reblog a post from here to twitter or insta or etc. The sites don't talk to each other that way. The only way to share something on tumblr to another platform is to share the link to the post, OR, to screenshot it here, and post it there, which leads me to:
Re-POSTING, which means taking someone else's work and claiming it as your own either on this site or on another site. It means screenshotting artwork or a post and making a brand new post (on tumblr/Twitter/insta/pinterest/facebook/etc), pasting the stolen content in, and sharing it as a post under your own username. Sometimes, people will do this and they'll add a link to the original creator of the piece. That's better than nothing, but still not ideal, because you're still posting someone else's content under your name to a different site without permission.
(Obviously, if you ask permission and the creator grants it, then that's fine.)
But mostly, people do this without giving any credit at all. Not only will they not link to the original post, they'll often go a step further. They'll cut out or blur artist watermarks, and they'll straight up allow people to think theyre the creator of the piece, etc. It is, in a word: theft. And it is, as you can imagine, very strongly disliked by content creators.
Reblogging = good. Includes credit to the OP, shares something you like to others who might like it, thereby bringing joy to the creator and to whoever sees and enjoys the post.
Reposting = bad. Theft. Stolen property. Cuts the OP off from their own creation. Discourages the creation of new content. Generally a dick move.