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?