Thanks for asking! Tell us why you posted the image with some context. Is it the facial expression of the person in the photo or something they're doing? Is there something compelling about art or photography? If it's a data visualization, what's the take home message? Imagine you're explaining it to someone on the phone. @micr0

@micr0 It depends. On the web an alt text must be concise. Everything that is important, makes sense, is mentioned somewhere in the text, is to be conveyed. but conciseness is first.
Here on social networks, I'd say, completeness is the first and even more important than conciseness. For instance, if you post a meme, describe it even if it's super lengthy. Like: "Three panels from left to right, on the first panel there is a man..." and on and on you go. It's important because *the image* is the unique thing you share, I have to laugh, to think, to be angry or emotional about *the image* itself, without any context basically.
Ask further questions, I'm glad to answer everything.