Furbooru and E621 are Boorus for furry art. On these sites, anyone can post anyone's art, with some restrictions that may vary, but the users are expected to provide artist names and sources. Any artist who has verified themselves can request a Do-Not-Post entry, which limits or completely prohibits reposts of their work. Although both sites allow 18+ users only... If you're a minor, writing something like "do not repost" on/under the art is about the best thing you can do... E621 also prohibits art made by users known to be minors, even retroactively (meaning that if someone was known to make a picture at the age of 16, this picture cannot be posted, even if they're now 24... Sounds like BS, but I don't make the rules).

"I don't like that someone reposted my art to Furbooru/E621, can they remove it?"
There's no convenient way to remove anything from these sites, UNLESS you have a DNP entry on there. Only verified artists can request DNP entries.

On Furbooru artist verification is simple:
1) Create an account
2) Click Artist Links in your profile
3) In the 1st field, type in your artist tag (e.g. "artist:stinkek"). In the 2nd field, insert a link to your profile page on DeviantArt/FurAffinity/Newgrounds/Instagram/etc. You can choose whether this link will be visible to everyone or to staff only. Double-check everything. Click Submit.
4) The site will generate a text code. Copy-paste it somewhere on the aforementioned profile page where a staff member can see it.
5) Wait... Once verified, you will see your artist tag and the profile page link in the Artist Links section, and you can remove the code.
6) [optional] Repeat steps 2-5 to list additional profile pages.

Creating a DNP entry on Furbooru:
1) In the bottom, click Do-Not-Post List. Click Create an entry. Click Yes.
2) Select a reason, fill in the DNP Request Form, click Submit Request.
3) Assuming the staff don't ask any questions, you can just wait... You'll know the DNP entry is active when you find it in the DNP list, or when you click your artist tag. It is technically possible to make more than one DNP entry if the reasons are different.

On Furbooru, images violating DNP conditions can be reported under Rule #1.

As for E621... sorry, can't help you with that one. And it allows c*b, which is blacklisted by default, but I'm still not sure you would want to go there knowing this.

"They should've asked my permission first"
Oh, I'm sure every artist is still alive and cares enough to log into their account they didn't use for at least 5 years to answer that. And if a site dies for any reason, any of its exclusives should be lost because the artists never said "yes".

Sarcasm aside, you should post your art to at least 2 websites, preferably those that don't turn everything into JPEGs. Instagram better not be one of them, unless you're okay with feeding your art and maybe selfies to AI.

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If DeviantArt allowed disabling comments on journals, I would've done it there as well. I wish they allowed us to disable Notes and private chats too, there are more bots than humans messaging me. But no, it's so much more important for DA to turn their site into discount Patreon where any talentless meatbag can sell AI-generated imagery, and have AI ADVERTISED too.