The first photo does more for your bookings than everything else on your site.
What makes a good cover
It shows what sets you apart, not what is prettiest in itself, and it must stay readable very small — a traveller sees it first in a list, on a phone.
What never goes first
A bathroom. A corridor. A wide garden shot with nothing to hold the eye. A dark evening photo.
The order of the rest
Tell an arrival: outside, living room, kitchen, bedrooms one by one, bathrooms, outside in detail, surroundings. Alternate wide and closer shots.
How many
Between twelve and twenty-five. Below that, a traveller doubts they have seen everything; above it, they stop looking. Show every room, including the small ones: a missing room reads as a room being hidden, which costs you more than an average photo ever would.