Several limitations on web images combine to produce unappealing results. The most stringent is that images displayed on a traditional monitor have 96 pixels per inch, no matter how many mega-pixels were delivered by the camera they came from. This constraint is very different from print photography, where pixel densities are usually at least 600 DPI. (Loosely put, dots per inch equals pixels per inch.)
By the time a picture is rendered in a web browser, there has been a reduction in pixel density, which normally involves pixel averaging, resulting in a slightly blurred, slightly hazy image. Typically, the web version of the image is saved using a default jpg compression of 85%, resulting in major loss of information and the introduction of jpg artifacts; these are most noticeable at the edges of flat surfaces, as around a window molding. A camera with a high megapixel rating may deliver good jpeg images despite information loss, but this defeats jpeg's mission of delivering small file sizes.
Websites that upload images often perform automated resizing and cropping, possibly with severe loss of quality. So it pays avoid such processing by uploading a pre‑cropped photo. In some cases, the expected size is well advertized, as in the case of a standard 512px X 400px real estate photo. In others, the best way to find out is to take a screen capture of an already posted image.
Beyond web constraints, there is the fact that cameras and people see differently, and most of what goes up on the web isn't coming from professional photographers. People know that buildings have vertical, parallel walls. Cameras don't. Inside shots taken at eye-level have walls that jut outward towards the ceiling. The eye automatically adjusts light and shadow to achieve an easily understood image. Cameras don't. Outside shots on a sunny day may have deep unreadable shadows. People automatically de-emphasize distractions of little interest. Cameras don't, accentuating power lines and other clutter.
Indoor shots are commonly taken with macro lenses that tend to scatter light, leaving a thin gray film on the image. They also tend to cause "fish eye" barrel distortion.
Happily, most of the above problems are readily addressed using modern image editors, albeit with some loss of quality. The exception is jpeg artifacts whose removal is difficult at best. For this and other reasons, it is much better to start with originals images that have high pixel densities. And when saving images in jpeg format, use the highest available quality setting.
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