Since almost all the art and pictures I come across on Tumblr have no alt text, I figure maybe people don't understand what it's for or know how to use it. Here's an explanation!
Some people (mostly blind people, but it can be useful for other disabilities too) use programs called screen readers to navigate the computer and read text aloud. That's why pictures need to have a text description (alt text), for the screen reader to read aloud. Alt text is usually hidden and just there for screen readers, though on this site, anyone can click the button in the bottom left corner to access it on any image that has it.
Alt text makes the internet accessible for anyone using a screen reader. I used a screen reader for a while, so I can tell you how much it sucks to try and use social media only for every post to be like,
"This is the funniest thing I've ever seen!" (image with no alt text)
"This info is so important, everyone please read this!" (image with no alt text)
And then all the responses are agreeing and everyone is having fun and being social, but you can't join, you have no idea what's going on, you're left out of the fun, the cool stuff, the funny stuff, the important stuff, all of it.
On this site, when you upload an image, just click the three dots on the bottom right corner and click the option to update image description. You can also add it later by editing the post, if you forget when posting.
I'm not the authority on alt text, so I can only give advice based on my experience as a sighted person and what people have told me.
One advice I've seen is to describe it like you're describing it to someone over the phone.
I think a good method is to start by stating the general, then moving onto details. I might start with, "Digital art of an octopus merman," and then go on to describe the pose, the emotion/facial expression, the body, the hair, the colors, the background.
Every screen reader user has different preferences, I've talked to people with completely opposing opinions. But usually someone can move on if they find there's too much description, whereas there's nothing they can do if there's not enough. So I lean toward over-describing.
Additionally, make sure you include anything referenced in your post. If you're just sharing a screenshot of a group of characters, you probably won't describe the shape of each of their noses. But if you say, "I love Bob's nose!" now you should still describe the overall picture but also include the shape of his nose in your description.
That also applies to things like, "This tree looks how I feel." If your alt text just says, "A leafy tree," that doesn't explain the feeling. "A leafy tree that appears to be hunched over with sagging branches, looking tired," is more helpful.
If there's text, include the text. I also like to give context (e.g. screenshot of a tweet). You can even use websites like this one to extract text for you.
Don't just put the word "image." Don't use alt text to credit artists/photographers. Don't use it for jokes, unless the joke is genuinely helping to describe the image.
And remember, trying is always better than not trying, even if your description isn't perfect!