Small question, as I don't know how Tumblr handles it. If you upload an image you're able to add an "Image Description" already. However, is it more helpful to add a second one in the actual description below it too (because that would be the one even those without screenreaders would be able to see)
Hey there! I'm guessing the built in image descriptions you're referring to are alt-text, which is meant to make images accessible for people using screenreaders. While they are a step in the right direction, they have a number of issues. Namely that they're only really useful to people who use screenreaders, which not all people who rely on image descriptions do, and that they're unreliable in function. Unfortunately they do not always show up or get added to posts properly, often rendering them unusable. Writing the ID in the caption is generally more accessible, and while you don't really need to write both, it wouldn't cause any problems if you did.
Feel free to reach out if you have any other questions!
I’ve also seen some people suggest adding a brief ID via alt text and a more detailed one in the body of a post. Tumblr has a 200-character limit on alt text, so it often can’t cover all the description people want, but something brief like “fanart of Jon, Martin, Sasha, and Tim at Martin’s birthday party” could help people with screenreaders save time by deciding how/if they want to interact with the rest of the post.