This seems a universal constant, not something unique to this site, and therefore I've refrained from posting it in the site related section.
You painstakingly get your image to just the right density, the shadows at just the right depth, the contrast ratio within the subject of the image* just right, and what happens?
The first thing people see of your image is a thumbnail on which the ratio between the shadows and lighter tones has been exaggerated, seemingly giving the image a dark, dismal tone. Adjusting, (spoiling), the actual image to compensate is, of course, not a viable course of action.
* The ratios within the subject, excluding the white background.