Why did the details and "color" of the picture change when uploading a file to the site?
I wonder... did you, by any chance, uploaded on a png format? From what I can remember (I stopped using the png format ages ago for uploading images), png can, sometimes, get varying levels of "posterizing" effects, not unsimilar to what happened to your render. Now, I'd sugest going for tiff files, as raw render, and run your editing on that, they have the most forgiving compression, and, then, go for jpeg. It seems like the most compatible option across OS, hardware and browser options
P.S./Edit
I can't type proper English today. My brain won't process that, apparently.