@killyourfm Be brief, talk about what's relevant to you. Context is key; what do you care about in the image. If there's relevant text in the image, describe that. And don't put in info that sighted users don't know (ie. "me and joe at the lake last september" or "made with midjourney") captions should be in visible text, not in alt.
https://webaim.org/techniques/alttext/#context
I blogged the basic rules a few days ago: