The exterior is the photo a buyer sees first in search results, and it is the one shot you cannot reschedule around the weather. A house photographed under solid cloud reads as grey, damp and slightly sad even when the property is immaculate. Replacing the sky puts back the condition the house would have been photographed in if the calendar had cooperated.
Technically it is a masking problem. The roofline, chimney, bare branches and any glass that reflects the sky all need to separate cleanly from the background, and the new sky has to match the direction and colour of the light already falling on the building. Get the light direction wrong and the image looks composited — a warm sunset behind a house lit by flat noon light fools nobody.
Choose the replacement to match the house rather than the most dramatic option available. A soft blue with light cloud suits nearly every property and nearly every board's rules. Heavy sunsets suit waterfront and elevated lots where the view is the selling point, and look odd on a suburban semi at what is obviously midday.
The giveaway in a bad replacement is a hard halo along the gutter or a tree reduced to a green blob. Edges are masked at the level of individual branches, so bare winter trees against the new sky still look like trees, and the roofline keeps the slight irregularity that makes it read as real.
A new sky also changes the ambient light on the house. Colour and brightness of the facade, lawn and driveway are adjusted to agree with the sky you pick, so a warm sky warms the brick slightly rather than sitting behind a building that is still lit for overcast conditions.
Nothing about the property changes: the roof, siding, windows, landscaping and neighbouring structures stay exactly as photographed. Only the sky is substituted, which is the distinction most MLS policies draw when they permit replacement with a disclosure note in the listing remarks.
Upload the grey-day shot you were not happy with and see it with a sky that matches the light.