Or should it describe everything?

It depends. If you want to make it good for everybody write a long alttext with everything but first on focus on what you want to present.

People that don't want to know everything can then skip the rest.

Alttext should describe as much as necessary and be as concise as possible. What that means depends on the picture. In most cases, it means describe the important bits, the reason(s) why you're posting the picture. If you're showing off a painting, it might mean going into detail on everything. It might also mean just "Spongebob looking confused" if it's a reaction.

Basically: Why are you posting the picture, what does the person who can't see it need to know?

Alt text is, at a minimum, "here's what you need if for any reason you can't see the picture."

What that means depends on what the image is. If it's a screen-shot of a post on a different platform, that minimum's the platform, any username or date, and the text of the post content. For a meme image joke, it means a brief description of the image.

Good systems also let you add more than the minimum, like the "mouseover" extras on XKCD, but these are extra. And they shouldnt detract from the image itself.

In socialmedia, should [...] ?

Oh, come on... social media is toilet walls. Everybody writes what they feel like writing.

Is Lemmy not social media?

Why shouldn't it?

What?

socialmedia

Needs a space in there, my dude. It's still two words.

You're not wrong, but you're not entirely right, either. Social media is the underlying tech or idea. It's broken into different communities. You gotta behave to the standards of the community or you're an ass.