Credit art when you use it. It isn't hard.
Since some of you on Tumblr, especially in the Avatar fandom, are unfamiliar with art usage etiquette, allow me, an artist, to direct you on what is considered ethical and legal usage of artwork you didn't make.
Credit. Shit. When. You. Use. It.
You do not have an excuse for not doing this, it's incredibly simple. Find the original artist (which you don't need to do if you're taking straight from the source) and credit them either immediately below the artwork or at the end of your post. No hiding credit in the tags.
"Credit to artist" IS NOT CREDIT.
Nor is any variation of it such as "not created by me" or "credit to [random pinterest account]. Credit is specific and direct. You do not see "credit to visual effects artists" in the end credits of a film. You see names. Online, you should always credit someone's social media handle.
Then do not use it. It's literally that simple. Don't know who made it? Either make an effort to find out or leave it alone.
If an artist says not to use something in a certain way, DO NOT USE IT THAT WAY.
Always check people's bios, watermarks, etc. to see if they list their terms of use for the art. If you're unsure, message them directly. "Do not repost" means DO NOT REPOST. "Do not use" means DO NOT USE, FOR ANYTHING.
People pay money for art, you know.
The art you carelessly download and use for your fics or roleplays could very well have costed upwards of hundreds of dollars. Ask yourself how fair it is for you to take it and use it willy nilly without paying, whereas the commissioner paid their hard-earned money to have it created. It isn't fair at all.
Other people's original characters are not yours.
Not everyone is okay with their original characters' likenesses being adopted for your own. Original characters can be extremely special and personal, it can be insulting and heartbreaking for their likenesses to be stolen and assigned to completely different characters. Ask yourself how that shit would make you feel. I can tell you first-hand it doesn't feel great.
Pinterest and other repost sites are plagues to artists, just don't use them for sourcing art.
Who cares? If they posted it online then it's free to use!
The law cares, and no, that isn't true. Art created by human hands has automatic copyright given to it. It is against copyright law to use artwork made by others in a way they have not permitted. Yes, fair use exists, but fair use is not a get-out-of-jail free card to use whatever material you find online. To be protected by fair use, your use of copyrighted material has to be transformative for the purposes of commentary, demonstration, or parody. Grabbing art off the internet to use for fics or roleplay is none of those things. It does not apply.
Are artists gonna sue you for using our art? Probably not, because we're all broke ass bitches who can't afford lawyers, but this is like, common decency, yo. Also, we can file DMCA takedowns against your posts / accounts for using our art without authorization, and guess what happens if you rack up enough of those?
If you really want to use a piece of art, consider just... reaching out and asking the artist who made it if it's okay to use noncommercially and with credit. A lot of the time, the act of reaching out and getting permission puts you in our good favor. A lot of the time, the answer will be yes. Just ask. We won't bite your head off. Just be willing to take "no" for an answer.
Thanks and have a sexy day