Image alt text: what it's for and how to write it
Alt text is the written description attached to an image, read aloud by screen readers and shown if the image fails to load. It is not a place to repeat keywords.
Write alt text as if describing the photo to someone who can't see it, because that is exactly its primary purpose. "Living room with large windows overlooking the lake, wood-burning stove in the corner" tells a screen reader user, and a search engine, what's actually in the image. "Beautiful vacation rental cabin lake view cozy getaway" tells neither of them anything real — it reads as a list of words attached to a picture, not a description of one.
Be specific to the actual photo, not generic to the listing. If you have a photo of the kitchen and a photo of the deck, their alt text should be different and should each describe what's actually visible, not the same sentence about the property as a whole repeated under both.
Leave alt text empty (not missing the attribute, but an empty value) for purely decorative images that add nothing if described — a repeated background pattern, for instance. For every photo that's actually part of your listing's content, write something a person would say if asked "what's in this picture?".
Name your image files descriptively too, before you upload them — "lakeview-living-room.jpg" rather than "IMG_4821.jpg". It's a small thing, but it costs nothing and adds one more small, honest signal about what the image shows.